If i knew he was going to turn top pair then i would have checked the flop as well.
However when i have a loose passive player i am just firing 3 streets here, you win a shit load more in the long term.
I think that's the advice the OP was looking for.
If I can pinch his hand for a minute, the problem that us weaker players have is getting value out of a great flop. Coz we're scared of making a weaker hand fold, then checking the flop OOP is the easy option, and it works as long as the villain bets out. But he's only going to bet out if he's either a perenial bluffer, or he's got a half decent hand himself.
In which case he'll call or raise any bet anyway.
My problem wouldn't have been betting out, but more HOW MUCH to bet.
After the flop he's got 3900 behind. I always try to get the money in before the river, so if he's chasing a draw, he'll be happy getting it in while he still has a chance. If he waits for the river and misses, I don't get all his $$.
With a pot of 1200, I'd want to bet out pot sized after the flop, so I could get it all in on the turn. But that sized bet would scare me. So I'd probably bet out with 900.
Assuming he calls, I'd then bet out 2400 on the turn, commiting him, and hoping he pushes over the top.
But like I say, I'm not a good player, and bet sizing is a huge problem to me.
Ignoring the result, and assuming everyone is happy to get it all in on this flop, what's the best way to go about it ?