Although going to the doctor might not be a bad idea, it probably isn't needed yet. Seeing as your BF is in his early 20s, puberty is ending and his sex drive is balancing out to the point where he doesn't have erections as frequently any more. It isn't uncommon to lose an erection during foreplay, just as long as it can be brought back for the main event if you will.

Try having sex at different times of day as well. I find that having a very set sexual routine e.g. just having sex in bed at night, starts to lose its magic. Morning sex, typically aided by a sleep erection, is a great time in the day to have sex for a man.

Oh right, I'll try that and see how it goes. It's pretty awkward, since we only really get to see each other on night times but we'll try. I hope it's not going to be a problem. He lost it duing foreplay earlier today and we tried before sex but no joy. He got a little frustrated about it. How long do you think this has to persist before it warrants a trip to the dr? I mean I hope it doesn't persist lol

Give it a week or so. Often, erectile dysfunction is psychological although there's a chance it may be physiological.

Hey, you might pay attention to some unrelated things. For instance, I used to get so turned off if my girl had say, a breath problem, my whole body would just shut down. I didn't want to be there and felt no obligation to make it happen. All I'm saying is at his age it is not a mechanical issue, it is a mental issue and for many guys all it takes is one little thing to turn them off.

No pressure will do the trick...

I think you are to aggresive and that turns him off, let him be the aggresive one let him be the man.