Redheaded Stranger,
First, I'm going to get all precious and say it's more solitude than isolation for me. Your bloke seems to be doing what I do.
What it feels like is an overwhelming pathological need for complete solitude.
And it's very democratic, this may sound trite but don't take it too personally it's avoidance of the whole human race.
What I feel at these times is a real need (just like hunger, tiredness, breathing) to be on my own, to get away from the noise, buzz and demands I can't meet.
After a day or so the engine winds down to sensible revs, I start to feel again and I'm not spending all my time avoiding, or, worse, in lock-down.
As to what he's feeling, well, probably not very much. The flight reflex has taken over, can't fight, the rules of engagement are changed, so it's time to bug out. Feelings, sadly, don't come into it very much.
And that's the bit that most people find so hard to understand, and I don't blame you! What he and I do is so completely opposite to normal human sociable instinct and affection that it must be very hurtful. If he's anything like me he's probably on a guilt trip as soon as he's gone but that over-arching need for solitude is going to win out. That also explains the lack of comms, the appearance of not caring, the disconnect. The best thing to make him want to come back? Reassure him that you're coping, that frees up a lot of headspace for calming down. If you can, try not to be hurt by the withdrawal, and if you succeed in that, let him know. You're doing it right with the short, simple & reassuring texts.
Agree with what QueensOwn says, too, don't get too worried by the signs of irritation. We've all got that exasperation with people actively chasing noise and trivia. It's not snobbishness or a superiority complex, he's just been re-wired, quite fundamentally, and those things are surplus to requirements.
You won't meet many materialistic blokes on these pages, it drives my lass bonkers sometimes.