He's hurting and terrified. Ask if you can go to either his appointments with him or for some couples counseling. He needs you, but is trying to hide from the horrors he foresees. It sounds like he's hiding from himself while he thinks he's just hiding from you.
See if he'll join the ACS Cancer Survivors Network Colon Cancer discussion group. Lots of people there have had colostomies, and are doing well. It isn't the end of the world, tho it certainly can seem that way.