Am I better off just assuming he made his flush?

Ultimate Bet Pot-Limit Omaha High, $.25 BB (8 handed) from (Format: Bet The Pot)

CO ($17.75)
Button ($27.50)
Hero ($24.25)
BB ($43.30)
UTG ($23.25)
UTG+1 ($27.70)
MP1 ($23.75)
MP2 ($29.70)

Preflop: Hero is SB with , , , .
4 folds, Button calls $0.25, Hero completes, BB checks.

Flop: ($0.75) , , (4 players)
Hero checks, BB checks, Button bets $0.75, Hero calls $0.75, BB folds.

Turn: ($2.25) (3 players)
Hero checks, Button bets $2.25, Hero raises to $9, Button calls $6.75.

River: ($20.25) (3 players)
Hero checks, Button bets $17.5 (All-In), Hero ?????

It's read dependent but in general you need to give credit for it, yeah. Way it's played out so far he could easily have a set but you can't even beat a weedy flush here, so GENERALLY unless he has previous convictions for bluffing you'd have to assume he's playing it pretty straightforward here.