He might donk the river with a Jack, but he'll also do it with all sorts of other junk you beat, and is unlikely to pay you off, so just call.

Preflop is good, flop bet is fine, I'd probably go to 4000, just because I don't like odd numbers. (???)

Turn check-behind is the scaredy-cat thing to do... and the smart thing to do after he called that large flop bet.

Very well played.