I'd leave his age out of it. That's a distractor. There are residents over 50, end of story.
What this is, is a complicated question about qualifying for residency in the US. This thread should most likely be moved to international, where such matters are discussed by people who know what they're talking about. That said, a couple of notes:
What matters isn't time on the planet, but time elapsed since graduating med school, ability to get that coursework evaluated, language proficiency, clinical experience in the US, and all the other stuff foreign medical grads need to do, which you can find on ECFMG.org.
It's one thing to have a fresh medical degree from outside the US or Canada, and try to get a residency in the US. It's a very different thing to bring a 15-20 year old degree from outside the US or Canada. I would expect it's difficult to get that vintage foreign degree evaluated.
In his shoes I'd start identifying some US residencies that don't fill. These are not going to be in the nicest places to live, they're not going to pay very well, and they'll be in primary care. I would then reach out to the directors of those programs, and start a conversation. Gotta get in their game. See the data on NRMP.org.
There are a couple of states that allow MDs to work as the equivalent of PAs while working their way into residency. Missouri is one. This could address the US clinical experience issue. There used to be a "fifth pathway" option at some med schools, but as I understand things, this is no longer allowed.
Best of luck to you.