when does the penis stop growing?

hello.
i have a boyfriend who is seventeen years old.
he isn't very well endowed in the penis area (let's say 3.5 inches), but he has large scrotum/testicles. i was wondering a) when does the penis stop growing, or should i just expect it to remain the same size at his age, and b) does the scrotum thing indicate that he's going to grow bigger?
by the way, sex isn't very stimulating for me. to put it frankly, it makes me feel like i have a big vagina, which isn't at all a pleasant feeling. i've been sexually active before, and i usually put ice up there to help heal the tissue that's been damaged during sex with a more well-endowed guy, but it doesn't seem to have helped with him. :/

also, it's not as if it's blocking our relationship (since we aren't exclusively sexual), i'd just like to know.