My boyfriend and I been together for 5 months we're both 21 we tried having sex plenty of times , he gets hard but then right before it happens he goes limp. He doesn't smoke or drink and is in good shape. It could be that he's nervous, but I just want to know I'm frustrated now.

Time he visits a doctor... lots of possible causes... most have no obvious symptoms... and some can be life threatening... at any age.

They say that the penis is the thermometer of a man's health. It could be psychological - nervousness, self-doubt - or it could be purely medical. I would talk to him about it, and try to get him to see a doctor.

Thanks, I feel like it concerned me more than it does to him

If its hypertension or diabetes.. he better start caring more than you do.

If its hypertension or diabetes..he better start caring more than you do.
I hope not, he tells me gets really nervous but its like really all the time that's just doing too much

I hope not, he tells me gets really nervous but its like really all the time thats just doing too much

Neither have symptoms until serious irreversible damage has been done... and why I suggest he see a doctor. Just because you are "in great shape" and "feel great" doesn't mean something serious isn't wrong.

I know guys that had and still have six pack abs into their thirties that have been under medication for hypertension since they were teenagers.

"Thanks, i feel like it concerned me more than it does to him"

Can you explain that statement a little better?
It can be interpreted to mean that you feel you are causing the problem
OR
Did you mean that it doesn't bother him that he can't perform, but it bothers you more?
Either way, it seems to me that there is more to this story.

Sounds more emotional than physical to me, because he can get erect.
Something is holding him back in his mind - fear about pregnancy (I assume you are on the pill, right, otherwise you shouldn't even be asking this), religious hang ups, or just plain not wanting you or any women or straight sex instead of something kinky. There's a thousand reasons.
If it doesn't bother him, then you need to decide whether to leave or stay. It isn't your job to drag him to doctors. Ask him if he wants to go, and if he says no or maybe or yes but does nothing about it, it's not up to you to do anything.