I think that's great that he has a girlfriend! About his voice, some guys don't change until they are 16. Their growth is a little later than girls. I would assume since you are already seeing changes in him, that you should just give it a couple years. He WILL grow, just takes time. But once his growth spurt comes, he will probably shoot up like a weed.
Just tell him not to worry so much, everyone notices more about themself than people would notice. And besides, everything he is worrying about seems completely normal. Anyways, hope I helped!