I predict that he has two pair and the nut flush draw. Note that, if he has two pair, that kills a lot of your redraws if he does hit the flush. If you throw in a gutshot, then you are only a little bit ahead:
http://twodimes.net/h/?z=428948
Omaha Hi: 820 enumerated boards containing Ac 6d 2d
cards win %win lose %lose tie %tie EV
As 6s 9c Ah 453 55.24 367 44.76 0 0.00 0.552
6c 5c Ad 4d 367 44.76 453 55.24 0 0.00 0.448
I am surprised at how much that gutshot helped him. Here are the results without it
http://twodimes.net/h/?z=428950
Omaha Hi: 820 enumerated boards containing Ac 6d 2d
cards win %win lose %lose tie %tie EV
As 6s 9c Ah 521 63.54 299 36.46 0 0.00 0.635
6c 5c Ad Jd 299 36.46 521 63.54 0 0.00 0.365
I think that I would just call him, here, and take a look at the turn. If it is safe, then you are putting your money in with a MUCH bigger edge. Also, if he really does have two pair, I think you get more out of him by waiting for the turn. Here is the gutshot hand, but after a safe turn card:
http://twodimes.net/h/?z=428956
Omaha Hi: 40 enumerated boards containing Js Ac 6d 2d
cards win %win lose %lose tie %tie EV
As 6s 9c Ah 29 72.50 11 27.50 0 0.00 0.725
6c 5c Ad 4d 11 27.50 29 72.50 0 0.00 0.275