I think it's obvious what he should do if he's smart. Focus troops in the southeast. Destroy the Azov Regiment. Take their uniforms, patch, flags, make a big show of it on all media. Claim mission accomplished. "We de-Nazified Ukraine!" Set up a base on the Azov, withdraw other forces to 2014 borders. Commit $x billion in rebuilding aid and payments to families in Ukraine, once sanctions are lifted and currency exchange returns to Feb 25th levels so they can afford it. Prosecute a few token scapegoats domestically for war crimes (anyone for whom the evidence would make it politically impossible not to).

Ukraine will never trust them. But it's enough to take the pressure off, and enough for him to remain in power.