Well, I'm not sure if you will be able to change his mind on the subject - well, you may be able to convince him that there is really nothing he has to be worried about, but it will take time for him to really accept it. He obviously feels 'threatened' for some reason or other and he probably can't just turn off those feelings just like that, even if you do convince him that he is wrong.
When you talk to him about it next, don't tell him that he is being unreasonable or that he's wrong and you probably should not tell him how much it's hurting you because he will just become defensive of his position. Start out by telling him that you understand that he is just so protective because he loves you so much and then try and put yourself in his position. How would you honestly feel if his best friend was a girl? What if tomorrow he came along with some girl in tow and spent a lot of time with her? Just tell him how that would make you feel and that you can in some way relate to what he's feeling - because you love him. Let him know how much you live HIM and not the other guys. Then you can tell him again how they really are just friends and that in time he will surely realise that. Don't press the issue too much, I wouldn't let it escalate into a 'either them or me' situation. Just keep reassuring him and hopefully his doubts will slowly vaporise. if not... ummm... I'm not sure what I can suggest then, but I'd just try to very carefully win him over to your view of things.