The man's whole schtick is puffing up his chest. That's what he does (and it means he's unlikely to back down; very bad look for a bare-chested man of action).
But pronouncing psychiatric diagnoses on him isn't very enlightening. If he's a megalomaniac, then half the world's governments are headed by people who are similarly mentally ill. Discounting aggressive foreign leaders as "mad" is bound to lead to incorrect conclusions and bad decisions.
He seems to be motivated by some pseudo-mystical beliefs about Russian national history and destiny. I imagine that by now, there are few at senior levels in the Russian administration that don't share his views. Oligarchs are a different kettle of fish; they're not nationalists, they're egotists. But from what I've read, oligarchs have nowadays been largely supplanted in government by old buddies of Putin from the intelligence services.
I suspect his generals will be the first to crack. They will be worrying about mutinies in their largely-conscript army. Their soldiers didn't sign up for a civil war against their "brothers" (which is what this war is, according to Putin). And generals tend to be practical, rational people.