I'm Jim. He may be struggling (unknowingly) with some backward feelings of guilt over having another partner. I'm not saying he feels that it's wrong, and from what you've written here it appears you are ok (supportive even) with the other partner. If previously he has always been mono, (or tried) and this is new to him, his body might be exhibiting symptoms brought on by his former taboos. Just a possibility and I wouldn't worry too much about it.
Best of wishes to all of you