5'4" and 120 pounds is big enough for about any bow that will go down to 26" draw length.
Take him down to the bow shop and see how many pounds he can comfortably pull. I was about the same size and drawing about 50 pounds at that age, but it was all I could manage.
I'd look for a lightly used Matthews DXT or another short axle to axle bow. Tall people don't think about how hard it is to shoot a longer bow out of a deer stand, but it makes a big difference for me. I would try to get him into a 60 pound limb that you can turn down till he works up to it. It won't take in but one summer of shooting to have it cranked down.