I'll play devil's advocate: as he is your eldest child, if he will likely be leaving home for college, you could just take him to whatever new pediatrician you choose for your younger children, if that doctor includes patients up to age 18 (which mine does), and if your son is OK with that. He'll probably use the university health service from 18-22, and choose his own internist when he finishes college. (Given the choice between a waiting room full of kids and one full of old people, mine would choose the kids.)
I find that it differs from place to place, but around here, "family medicine" practices are rather rare, so I was never able to use that option. DH and I don't see the same internist, either, as we have different insurance policies; I carry the kids, and he has his own company plan.
PS: I have a boy and a girl, and while they both saw a male doctor as small kids, once my daughter (the younger one, 16) reached puberty she asked to switch to one of the female pediatricians in the same practice whom she had seen a few times over the years for sick visits. It isn't that she doesn't like or trust her original doc; she still sees him if her regular pediatrician isn't available for sick visits, but she prefers talking to a woman for her physicals and all of the accompanying discussions of sexuality and mental health issues. She does regularly see other doctors who are male.