He is now in, he can't get out easily. Obviously he needs to negotiate, but its clear that the Russians need Crimea/Sevastopol, that's non-negotiable, and the Ukrainians didn't compromise, they even stopped the water supply for the Crimea.
The Russians demanded at least autonomy for the Donbass, but they never fulfilled the Minsk agreement and shelled the "republics" constantly, threatening to eliminate his allies by force.
Putin and basically all important Russian leaders said that a NATO membership for the Ukraine is thick red line, but the West never stopped to move forward in that direction. They even motivated the Ukrainians to go on with the provocations and not compromise with Russia.
I do understand the Ukrainian side, but the current borders are ethnically and historically artificial, basically wrong. It became part of the Ukraine very recently, with a transfer within the Soviet Union:
https://en.wikipedia.org/wiki/1954_transfer_of_Crimea
Crimea was always strategically extremely important and Sevastopol is the military and civilian door of the Russians to the Mediterranean. They can't give it up just like that, but the Ukraine never compromised and the sanctions didn't stop, while the water supply was blocked and this ruined the region.
The Ukraine need to, at least, compromise about Crimea and the West needs to aknowledge this. Of course, Russia might be forced to give this up and lose, but only by very brutal and very dangerous means. This is and always was insane. That the Ukraine could become a NATO member is bad enough, but with Crimea, that's just too much by any means. This cripples Russia for the future in a way which no Russian leader which cares for the future of his country could just accept without a fight.
Now Putin risked everything to force the Ukraine to a compromise, he is all-in. There needs to be a compromise with which Russia can live, or we don't know where the escalation might go. And I repeat that this would be a just thing, that the Ukraine in the current official borders, with the Crimean peninsula, could just join the NATO without Russian resistance is absurd.
Any Western state or leader which pushed the Ukraine in that direction must have known that this would escalate at some point, this was a wanted a conflict, nobody can be that stupid.
The main thing I would tell him is obviously to accept a second best compromise and peace and keep his fingers from the button as long as possible, as long as Russia being not directly attacked by a foreign power, he has no justification whatsoever to even think about that desastrous option.
But then again, the Ukraine and the West need to compromise with giving up on Crimea, that's the minimum prize. Anything else is just madness and every reasonable historian, military strategist and politician should know that. Crimea and Minks agreement, autonomy, possibly neutrality of the Ukraine.
That should have been the compromise from the start, but the Western powers pushed Ukraine to not move one inch in the Russian direction and even encourage them to continue with the conflict in Donbas being force. This was a blatant provocation and break of past promises to the Russians, this is not just an issue for Putin, but would have upset any Russian leader which tries to keep his country and people independent.
I honestly didn't thought that Putin would attack the whole Ukraine, but rather use a limited operation for getting water supply and a connection for the Crimean and preserving the Donbas. That he was going all-in like came rather unexpected. But it just shows that he plays everything with one big card and this is extremely dangerous, but also kind of logical since the West would have done all the things it does regardless of whether the operation would have been more limited or not. Which is yet another problem with the Western measures: They said they will hurt Russia if they cross the border regardless of how they do it and where. So he just don't cares any more.
And that's typical for him and the Russian leadership: If you don't give them options, they will go even further. The less options they got, the more radical the solutions will get, which is exactly the situation of which I warned before and which might escalate even into a nuclear war in a worst case scenario.