A 16-year-old is too young for any 21-year-old, regardless of gender or level of social skills.
IMO, sufficiently weird if I were the parent of the girl - heck, probably if I were his parent as well.
But I'd find it sufficiently odd to be problematic for most high schoolers to be dating someone he/she did not at least meet in HS. This wasn't uncommon in my high school years either.
So the extreme would probably be for a HS freshman to be dating a HS senior - then the next year you could have a college Sr. Note this was in the early 70s, and a frequent complaint of high school guys was that a lot of the girls were only interested in college men.
Sure, 5 years down the line a 5-year diff wouldn't matter. But I would likely share the guy's parents concerns of why he isn't seeing girls his own age.