I think their point was: even if you're uncomfortable, what are you going to do about it? Interfering in any way - even just suggesting that this relationship is a problem - is controlling.
As another reply said, if you want to say something like "I'm not implying anything about this guy, but it does remind me to make certain you know the signs of predatory relationships", that's probably a good thing.
Just know that your son may react defensively at the perceived threat to his first meaningful friendship outside his home town (even if you're not actually a threat), and you have to let that be okay.
For the record, I'm 40 and have had friendships that started in my early 20s with people much older than me, and am currently friends with some kids in their 20s. Especially for introverts and people with niche hobbies, there's a lot more care for shared interests than social norms like age gap.
..... Sounds cool?