sounds like it could be performance anxiety, he may be worried about the exact same thing you are talking about, he could be worrying that he's not satisfying you or 'good enough' for you therefore putting pressure on him, which isn't ideal for sex let's be honest. It took me a while to get the lad up on my first time, and since i'd never done it before obviously I had that pressure of not knowing if I was going to be 'good' at it.
The best fix for this may be to just talk to him about it and communicate with each other, since that is the most important in any relationship and even friendships too. it's important to note that just because he's struggling to keep the fella up it doesn't mean he's not in love with you, or doesn't want it. There are a number of things it could be but like I said it's best to just have a chat about it 1 on 1.
And if it continues to be a problem, a quick fix can be the use of Viagra or other blood flow enhancing medicines, but obviously use these at your own discretion and I would advise you to only use it if your bf is 18 or over just because that's the legal age of usage. I don't condone under age drug usage but like I said its at your discretion and is not always necessary
I hope you and your boyfriend work it out and live a healthy sex filled life!