Were both testicles undescended?
It just so happens that essentially every guy I've dated had one undescended testicle when they were born and had to get surgery. Almost all of them had a testicle that was significantly smaller than the other one that was descended naturally. None of these guys had higher voices or effeminate features. One was actually quite muscular. It just seemed normal that the testicle that was undescended was smaller than average. By that logic, I would guess that if both were undescended they'd probably be a bit smaller than average. If you're really worried, talking to a doctor in person is probably your best choice so that he may conduct a thorough investigation. Best of luck with your son, I'm sure everything's normal.