[MRBAA]

In the game structure you describe, his starting hand selection isn't that bad (although it is too loose). That's because the ante is large relative to the bring in and opening bet, so it's worth staying with weaker hands to try to win the pot.

That said, in the hand where he had buried sixes and the rivered straight one, I think you played fine. You were a favorite, but he outdrew you. Happens all the time. If you want to limit your variance against this player, just call instead of reraising. But if he's as wild as you say, you'll be missing some opportunities to value bet. As I'm sure you will agree, this player will eventually lose his money. In the meantime, he can be tough to play against, though.