Gently, you can't do anything much at all about his triggering. It's his problem to deal with, just one of the consequences of cheating.
I think what you can do is limited to 1) bringing it up in MC, 2) urging him to seek IC with a goal of becoming a great partner and resolving his guilt, 3) be 'there' for him while he triggers, and 4) ask him if you can do something to help and doing it if you want to (and not doing it if you don't want to).
Treating him as a responsible adult who can solve his problems is a great thing you can do for him, and it's really the best thing you can do for him, too. JMO, of course.