Is the river a bet or is it too thin vs the standard fish? without the possible flush on the river it'd be an easy valuebet but with the flush I'm actually not sure -.- ...

I also don't know if he would jam the flush himself. I think it differs from fish to fish in my experience.
With his remaining stack I think he is more likely to jam the flush himself than check/call it... If he had like pot left or more than that I think people lean towards check/jamming or check/calling the flush.

But he could also play a super tricky set or some random twopair so it's really thin^^ and I'm not even sure if he calls worse except AJ or AT