Hello and welcome. Thanks for your question. Most likely your son has regular annual check ups with his doctor. At this time, they should perform a brief check of his genitals which is just visual. I would ask at that time. It doesn't appear you are asking about testicles that haven't ascended into the scrotum. https://www.webmd.com/parenting/baby/what-is-an-undescended-testicle#1. This is not your concern, correct? That again is normally addressed if the testicles do not drop on their own younger than the age of your son. That a doctor hasn't commented on this would be a good sign that he is actually normal and developing at his own rate. But do follow up with your doctor at his next well visit.
Yes, I agree. Have you discussed this with his pediatrician? Likely it is going to be just fine.