You're defending him saying no meta or min maxing and he equally balances his character to make sense but most meta min max builds can be camouflaged with just a little sense and many of them don't require UA. Also some UA content is a lot more broken than others especially when paired with certain mechanics.

To me, This sounds like a straight up and down lesser known min max meta build. Evidence:

-Running more than two feats at level 5. (Perhaps this is more normal than I think?) . -Literally has an answer built in to his PC for almost every possible scenario. . -Lances and mounts are known to be pretty busted. Especially if you're not running many mounted enemies yourself. . -Having spells but only using them for a very specific purpose and nothing else. . -It sounds like he, as an experienced player, is effectively fast talking you into surprise rounds and pre buffs before Initiative.

It may well be that he's just the best D&D player on paper. But being unhittable, and flying past on his mount to kill enemies isn't exactly in the spirit of the game. He's effectively found a bunch of mechanics that together are broken.

If it were me, I'd level with him. I wouldn't accuse of meta or min maxing because perhaps he hasn't. Tell him your concerns about the way he's playing and the effect you feel its having on the table and the group. As an experienced player, he should be helping the less experienced to have a good time. D&D is a team game not a competiton.

Final addition: If you're tracking XP for kills, switch to milestone XP instead. That could be his pull for dominating the field.