He will probably be growing very slowly for the next 3 years, but I wouldn't expect him to grow more than an inch or two, but he probably will fill out a bit.
I'd stick with the one you have. If it's only a tiny bit big then he'll more than likely fill out & grow in to it. I've always fitted my rugs slightly too big to avoid them being pulled up over the horse's rump when they're grazing.
Just to add, I'm talking an inch or so too big, not two foot too big!