The two big factors are:
1. Has he bet out strong with flops that hit him in the past?
2. Has he bet out strong into drawing boards to get draws out?

If the answer to both those questions is "no" I put him on AK, KQ, QQ, or perhaps just a flush draw (probably A high and maybe with the pair to boot). I think AK still folds. JT calls IMO. If the answer is "yes" to either question, and especially both, get out.