6 handed, 1/2. folded round to button who makes it £12 (£500), guy is reasonable, fairly tightish. You call from the SB with

(500). BB, bad fishy (50) calls.
Flop

As flop is dealt tight guy insta looks at flop then looks at chips, almost in a fluid movement. You check, BB jams £40 into £40ish. Button isolates to £100.
Best course of action is?