It's good to call the vet in, at least to find out what's wrong. Talk to her about the problem medicating him.

I'm so sorry he's not doing well, and that You both are still grieving Dorian. Keep talking to Max, even though he's not responding. Whistle at him if you can. Let him know he's still your flock member, no matter what. That's all you can do besides the vet, and loving him.

Sending hugs your way, and prayers that he turns around and gets better.