Pretty much the only thing you can do is say "Is there something I can do for you?" in the sexiest way possible.
There are generally three reasons for what medical professionals call "nerf schlong": his penis is out of proportion to his blood supply, he has something medically at issue with his cardiovascular system, or he's trying to force arousal and needs something, but is unwilling to ask you.
The first two out of three of these problems are really up to a doctor, who in the former will prescribe Viagra and in the latter will tell him to get a gym membership. The third one is usually a matter of a request being seemingly too kinky for a guy to bring up, so he tries to force the issue instead of being honest.
This is not to imply he'll be hauling out a gimp suit; when I've heard about this, almost inevitably it's about a person who doesn't know much about sex and thinks asking for, say, doggy style is indescribably weird and wrong.
But, in a non-sexual context, have an honest conversation with him. Don't frame it as underperforming in the bed; frame it as you're worried you're not doing something for him that he'd like, or that you're concerned about his health. Don't force it, let him tell you. Once you figure it out, it'll be smooth, er, hard sailing.
Wait wait wait, you forgot reason number four: ALCOHOL!!!!
and lots and lots of bike riding...
as well as lots and lots of bike riding
Hahaha thanks Dan!
I asked this question and .. it was only in the beginning, my thoughts were going towards because i think he was nervous.
I felt it wasnt helping just staring at him so i did ask if i could do anything (probably not in the sexiest way possible haha) and he DID tell me what to do and it went a little up and down from there, the next time we did it there were no problems at all.
I just thought id ask for future reference what to actually say to make them feel less nervous ... and asking what you can do for them is the answer i was looking for, i'll keep it in mind ;)