My wife & I have an 18-year-old son. He started university in September, studying medicine. There he met this 41-year-old man - a classmate, and they became really good friends. This man has just started university now at this age because apparently he was born poor and in the first half of his life was focused on making money, but medicine is a dream of his. Son has always been an introverted, shy, socially awkward kid with little friends, but now goes out often. Honestly my wife & I are uncomfortable, and we can’t help but side eye the dude.
If you go to uni, hanging out with people aroind your son’s age is implied. I bet the guy might feel out of place as well - but consider he is now living his dream of studying in university. He is surrounded by 18 year olds and he has two options - be isolated for nothing but his age or make friends and experience life he couldn’t experience when his age was appropriate for it.
It is likely your son and this guy just clicked. When I was around 16 I had a great virtual friendship (played World of Warcraft together, only met once) with a guy in his 30s. We also just clicked. I’d say he was my best friend in that guild. We are not in contact anymore but I still remember that friendship fondly.
I understand your concerns as a father and I’d recommend discussing it with your son with open mind. Friendship is just friendship, I don’t think there is an age limit on it.