luls.. Will everyone stfu about the "bet?" stuff? OP obviously knows it's a bet and said so himself in the OP. He asked specifically about betsizing.

I'd just go ahead and bet like $1.50ish. He's obv still stacking off with his nut range, and given stack sizes being deep he might be overzealous and think he has good implied odds with flush/straight draws. Also given he is loose/passive, these tend to be the players that are calling stations, so he isn't going to differentiate much between betsizes, and likely doesn't even consider pot odds/implied odds/paired board at all, and still calls his flush/straight draws.