When I was at my angriest and/or trying to go dark, I was the one ignoring his calls or texts. This lack of contact doesn't startle me. We've gone a long time without seeing each other, too, but we've never gone as long as you and your H.
You're so right about their getting in their isolation groove. My H can be more exhausted from a day's work than most others. So frustrating because it's so obviously depression.
I can't imagine that he is energetic at his mother's. I can envision him lying in a state of repose a great deal of the time. I can also picture him holed up in his old bedroom, staying on the computer or playing on his iPhone. It's so hard for me to think about his isolating himself. I wish there was some way I could help him, but I know that I can't do that right now.
Do we just leave them to wallow in their depression???