Hi,
I'd usually 3-bet that SBvsBU, flatting is fine tho.
On the flop I'd check/call after flatting preflop (since basically I flatted because I didn't think he was loose passive enough to call a 3-bet with a lot of worse hands, so chances are he's aggressive enough to fire at that board) to get a c-bet out of his air hands, if he has Ax/clubs he'll c-bet anyway most likely. Maybe he checks behind his weakest gutshots/9x/88-66/5x hands sometimes that he'd call a donk bet with, but those hands usually won't call 3 barrels anyway so we can always get one street out of them on a later street.
Turn is fine, he could have 9x sometimes but he'll still have Ax/clubs a bunch.
On the river it's really hard for him to have air to bluff with after calling 2 streets and I don't see him raising Ax there ever. I doubt even 9x would raise that river after only calling the turn. He probably made the flush exactly like his betting says he did. If you've noticed him making plays that make absolutely no sense I don't mind looking him up sometimes with those odds, otherwise NH overall.
Hope it helps.