IIN that my boyfriend doesn't make any noise when he cums?

Some guys are like that. Some guys are generally taciturn. Some guys might feel that making any sounds during sex would be animalistic or some such shit. Hell, some guys could be so stupid that they believe that making any sounds would be a sign of weakness or unmanliness.

However, I think what it very often comes down to is that the vast majority of boys masturbate from a fairly young age, they know that they'll be caught at it by their parents or siblings if they make any sounds, and so they train themselves to stay silent.

Not making an effort to break that habit is really pretty stupid though. Anybody with any sense realises that if you're engaged in any sort of joint activity with another person, things generally work a lot better if they both give feedback to each other.

You could try talking to him about this, not from a point of being critical of his behaviour, but from the point of saying that you want to make him feel good, and it's impossible for you to know what he likes and if he's enjoying things if he doesn't give you some hints.