Reasons for jamming - 

We block AA and we have to assume he 3 bets the flop with AA a lot of the time so we can remove that combo somewhat. If he does get to the turn with AA this way, i don't think he always decides to CR it either, even though given our blockers he will have AA + NFD about half the time.

77 unlikely although possible, AcKc77 or something. 

How he plays his 45 I think is a big key to this hand. He doesn't raise pot on the turn. His sizing is going to leave $769 behind into a $977 pot on the river. He could, if he raised pot, leave $692 behind into $1131 (I think I got that math right, point is he doesn't raise pot which if he is going to raise with 45, i assume he would). Also I think a lot of his 45 combos just call the turn. 45 with a flush redraw I can definitely see him raising, but naked 45 I think just flat calls a lot to avoid being in check / decide for almost 400bbs spot on a lot of rivers.

He can also have all of the huge draw combos with A789, AK89, A89T, although the weaker parts of those probably just flat call the turn. He can have double FD hands as well. AcXcXhXh

Reasons for folding - 

Its 500 bb's and we could be almost dead. If he has a monster draw we are only a 60-65% anyway so we're not that big a favorite.

Reasons for calling - 

I didn't want to have to make a decision on all the bad rivers as well, it also gives him the chance to check / fold some of his draws that I want to get the money in against now. I could definitely be convinced that calling is the best play though.

Thoughts?