What do you mean by tiny? Have you ever seen him erect? Probably not. Erect size and flaccid size are normally unrelated.

I do not believe premature birth affects penis size.

in general, penis size can be most relatable to stature. If he's a small boy, he'll most likely have a small to average sized penis. He also most likely hasn't hit puberty yet, and that can change things in the course of three years.

I'm with eiri. I don't think premature birth has anything to do with penis size!

No, and it will change with puberty.

You haven't mentioned your concerns to him, have you? There are far too many men in the world who are obsessed with their penis size.