As a 17 year old guy who has this 'problem', my advice would be try your best not to worry about it, and certainly don't think this means your boyfriend isn't attracted to you. I love my girlfriend dearly, and attraction and arousal are some of the many highlights of the relationship for me.
The problem for me predominantly lies with excessive and vigorous masturbation (once or twice a day is certainly common for guys my age). Men have a tendency to be quite vigorous when getting themselves off, and they don't need to use many muscles to do this (standard forearm - bicep - tricep interactions). We have to work a lot faster and use many more muscles to get anything close to the same level of stimulation during intercourse, which in itself is distracting, hard work. Yes, sex is certainly more of a turn on, but because the penis is so desensitised from the 100mph speeds of masturbation (okay, not THAT fast) we are turned on, but we aren't finishing. This brings me to the whole other problem of the aim of sex. Guys repeatedly have drilled into them through porn and friends that they've gotta make their partner cum first, and then they have to cum hard too. Sex no longer is about having fun together, it's winning or nothing. This is a problem for many reasons: there is an OBJECTIVE to finish, there is an OBJECTIVE to finish LAST, there is an OBJECTIVE to be mind-blowing in bed.
Needless to say, at best objectives are decidedly unerotic and at worst they are a distraction. I find that when I'm having sex the thought "how/when/where am I going to finish?" frequently crosses my mind. I might notice the feeling that I'm close, I normally focus on that feeling, and then suddenly the sex turns into an objective based activity full of 'methods' and 'timing' about how to cum, rather than the fun, intimate and rewarding 'bonus' an orgasm should be. As a result the feeling disappears and both partners are left disappointed. It may damage your ego if this happens, but trust me; it's on a level of emasculation on par with erectile dysfunction and premature ejaculation. Indeed it's the third most common sexual disorder in men; scientifically referred to as Retarded Ejaculation. I think there is a misconception in male communities that RE is a blessing rather than a curse and we should stop complaining. Unfortunately those of us who have RE can feel like we're missing out (well, we are..) but there is very little research into medical help for RE as a result of this misconception. Because sex then becomes an ego damaging activity, some men turn away from sex in self defense. They feel inadequate as lovers and as men.
Now, I have no idea how your relationship is with this man, and I won't presume to guess. Instead I'll just let you know about my continual efforts and personal experience. I had a traumatic first relationship which had a lot of emotional trauma behind it, which was followed by a stressful very objective, sex based second relationship. My current relationship is wonderful; it's only minor issue being my Retarded Ejaculation. To clarify; I am able to cum by myself, and I can cum during sex, although it isn't common. The sex is the best I've ever had, it's just that because I can only see my girlfriend on weekends (what with the **** load of school work and ongoing exams at the moment) I find myself being very horny during the week too. ****ing helps to relieve stress from all aspects of life (particularly school), it helps you focus on work as well as acting as a sedative helping you get to sleep if you find yourself pondering quantum physics in your bed. Because there's a combination of a lot of stress and testosterone during this week where I can't see my girlfriend I get myself off once a day on average, maybe more, maybe less. Because masturbation reduces testosterone levels (which is the intrinsic sex hormone in men), sexual intercourse can suffer as a result of frequent, vigorous masturbation.
I'm currently trying to remedy myself by simply cutting down on the amount I masturbate from daily, to weekly, to eventually not at all. Simple really, and in all honesty it'll probably be the most effective thing your boyfriend could do. He will get his sensitivity back and have higher testosterone levels which will improve his ego and his sex drive. Also, because testosterone is linked to exercise, I work out every now and then doing short bursts of weight lifting, rowing and push ups. Exercising in ways that increase his muscle stamina of muscles involved in sex (by doing push ups, sit ups, planking etc.) will reduce the amount of 'work' he need to put into sex, thereby reducing distractions and strains of sex. His body will improve at the same time, again improving his confidence which will only make him more happy to get naked with you.
While this is going on (because it will take time) make sure that when you have sex, you appreciate that he doesn't need to cum to enjoy himself (he certainly will) because your ego is important in this too.
I also recommend that while he's cutting down on masturbation, he adjusts his method of masturbation from the standard full "fist" to just his thumb and index finger. This will reduce the amount of desensitization he will experience.
Ultimately his goals need to be minimising the friction his penis experiences when he's not ****ing you while simultaneously maximising his testosterone levels and ego, and your goal needs to be creating the pressure free environment you seem to be trying too. It will take time, but you will both start noticing improvements. Through positive reinforcement it'll only get better.
As a final couple points, for extra effect try having sex spontaneously, ie. don't meet each other knowing you'll have sex. Little, super high quality sex is better than loads of disappointing sex. Some couples have a tendency to fall into a boring rut where sex loses its mystic and appeal. In teenagers like us, that normally results in the relationship falling apart. Finally: use of condoms. Condoms are a turn off for most guys I'd say, unless they find themselves too sensitive to have sex without a condom (people with Premature Ejaculation). You say you're experienced, so I don't doubt you know this. Perhaps you're ready to use the pill or other female contraceptives that don't interfere with sensitivity and get rid of the condoms, if you haven't already that is - just a thought.
I wish you both the best of luck, I'm still having trouble with the whole reducing masturbating thing myself, especially with all the exam stress, but it's the best way that'll yield the best results. There's no doubt in my mind that when I finally stop I will be firing loads like a machine gun.
Man, I hope you're still looking at this thread, this took ages to type