3rd nut flush faces river donk that makes little sense

Villain looks nitty (11/7) but only 40 hands yet.

He tanked before calling the turn.

Vs. a fish, I'd expect some ridiculous 5sXx type hand, but if this guy really is nitty nothing really makes sense, other than maybe KsQx, or a very unlikely 78ss. I guess a slowplayed T9s or 99 could think I might check back a non-nut flush but call a donk. I don't really expect him to have floated the flop with AsKx or AsQx and I don't see how he can ever have As9x or AsTx pre. He could rarely be slowplaying KsKx or AsAx pre but I can't see how he ever flats this flop with them.

Thoughts?

Poker Stars, $0.10/$0.25 No Limit Hold'em Cash, 6 Players
Poker Tools Powered By Holdem Manager - The Ultimate Poker Software Suite.

BTN: $33.71 (134.8 bb)
SB: $10.62 (42.5 bb)
BB: $30.94 (123.8 bb)
UTG: $28.19 (112.8 bb)
MP: $31.63 (126.5 bb)
Hero (CO): $82.27 (329.1 bb)

Preflop: Hero is CO with Q J
UTG raises to $0.75, MP calls $0.75, Hero raises to $2.50, 3 folds, UTG calls $1.75, MP folds

Flop: ($6.10) T 9 T (2 players)
UTG checks, Hero bets $4, UTG calls $4

Turn: ($14.10) 2 (2 players)
UTG checks, Hero bets $5.50, UTG calls $5.50

River: ($25.10) 6 (2 players)
UTG bets $16.19 and is all-in