Have him come in here and ping me his MC username personally. (TBH I'd be ok with making an exception but it seems unfair to everyone who voted "No" that I just steamroll the decision on the first chance.)
Hiccups perhaps but not major problems or anything. It's just that the server is specifically for PPCG/SE members. (It's also less hassle for me if I can tie every MC name to a SE name.)
@TheNumberOne (and @East) I have whitelisted a few people without SE accounts but I'm considering not doing it anymore since it kinda goes against the idea of the server. Here's a poll so we can kind of officially decide:
I can think of a few reasons. 1) Playing on this server feels like playing with friends because we know each other through SE. 2) People with SE accounts have this room for communicating, so problematic behavior can be corrected and events can be planned somewhere other than the game (and it doesn't have to all be at the same time). 3) people with SE accounts are slightly more likely to be mature, constructive Minecraft players, and if we allow just anyone, then we lose some of that control.
For point 3, I refer to the issue we had with Kenny. He does have an SE account...well, it's an SO account, and it has 18 rep currently, which is like the bare minimum for joining.
As far as I can tell, we only have one player that doesn't have an SE account, and I don't remember there being any issues with them, so it's not a hard-and-fast rule.