I'm not sure what you are trying to say.
But I will try to answer various ways:
1) you can never be 100% sure that there aren't any sperm when a guy comes - even pre-cum lubricant can contain sperm - and yes even that little bit can get you pregnant.
2) holding back won't do him any harm
3) guys can orgasm without ejaculate
did that naswer the question?

Hmm..Kinda


but what i'm saying is that...When he's abt to cum. He stop...Adn squeeze his penis very very hard.....But that didnt stop him come cumming. But nothing came out.... :roll:

When you are saying "cumming" you mean having an orgasm?
Because men can and do have orgasms without any ejaculate coming out.
It's probably best he doesn't do this every time, but it won't do any damage.

I have a friend who doesn't like to get semen everywhere when he masturbates so when he reaches orgasm he holds the foreskin tightly over the end, then goes to clean up in the bathroom.

It's not good to stop ejaculating when a male is going to have orgasm.Besides,i enjoy cumming...

Trust me you won't get an overload of sperm nor will you balls explode :p
as I say probably best he doesn't do it every time but to avoid a messy situation...