If he has IMHA work very very very closely with your vet on treatment and push for treatment NOW.
I'm not saying this to scare you, you already Googled it (I made the same mistake at the time) so you know this. That is what Happy died of. The e-vet he was at when he collapsed tested for it, gave meds, and said the regular vet needed to development a treatment plan for it, but then the regular vet just... didn't. And my sister is not a very assertive person so she didn't push on it either. And a week later he was dead. Of course he was in really bad shape when they found out he had it so it's not the same, but if there's stuff indicating he has it now, don't let it go. Be assertive, definitely.