CS, I have thought about this as well; I think the high-energy types have "low" periods, but overall they are the ones that went quickly, DID stuff about sorting their new lives, etc. This may be a spectrum as well, from the lowest lows (those who just lie on the sofa an moan) to the most manic monsters.
I've sometimes even wondered about mine, but then look at what all he's done and think "goodness, that can't be low energy". The new jobs, the toys, the trips, the women..... even though I've seen him be very depressed during this process, he always "does" something -- never sits there and contemplates (or if he does, I don't see it). So the last time he broke up with an OW he promptly took himself off for 3 weeks abroad, for example, before going right out and finding the next one.
So by that reckoning yours would still be high-energy (or on the higher side, at least), even if he hasn't been horrible monster, filed papers, etc.
You may well be right when you say that he realises that he has messed up but doesn't know how to fix it; he has to go through this phase as well; if he's stoic then that's part of it.
I only say that because way back when, in my "early life" crisis, I went through that -- and at the end I realised that I COULD fix it, and then did, in a hurry. But not until I decided to myself.
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