I don't have Chrohn's, but I have a condition that causes me extreme fatigue and lots of pain. I have dated several people in the past, and my current partner has been the most supportive of my condition.
I don't know if you have AS or if you partner does or what, but maybe think of it this way. If you do have AS, maybe you feel sometimes like your significant other can't fully understand you. He probably feels the same way about his illness. Having a chronic illness can make you feel pretty alone, because no matter how much your friends try to learn about it they can never actually experience it. Is it the helping him when he needs it that gets to you, or is he being difficult? If you just don't want to deal with the things that go along with his illness like helping him, then it's ok to get out of the relationship.
This is just from me personally...I'm not saying he feels this way. When the pain and stuff gets really bad, sometimes I consider that my significant other might want to leave me because I know I can be sort of pitiful when I'm sick. If I get down about that, I have a tendency to get sort of annoyed with her and I become a little distant. I try to do more things myself that I'm not capable of doing and make myself sicker, which makes me become more easily annoyed with those around me.
The best advice I can give is communicate with him as clearly as possible and be honest. Don't walk on eggshells all the time. Try to see things from his perspective, and realize that you won't be able to fully understand how he feels all the time. Sorry that's pretty vague advice.
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After a time, you may find that having is not so pleasing a thing, after all, as wanting. It is not logical, but it is often true.
--Spock