Ukraine
I don't think Belarus is a safe haven. It's a suburb of Moscow. He would need to go to Africa somewhere.
I don't believe anywhere on this planet right now is safe for him, personally. He could flee or try to hide in some remote, heavily-forested area in a nation hostile or neutral to Putin's Russia (like India) and his FSB agents---somehow, someway devising some halfway ingenious/insidious plot to kill him in a dramatic fashion, or just slowly poison him and maybe he realizes it when its too late to reverse it.
If Putin wishes him dead, especially now after going against him, after losing his nerve, getting so irritated and enraged at the pace, deadly slog, death and destruction of a war that you had serious reservations, doubts beforehand and these fears have become manifest due to pitiful fighting performances of Russian troops, war crimes, mostly committed by your paramilitary organization, and well over 100,000+ soldiers dead and wounded (and still counting), Putin will never forgive him and will order his FSB to "liquidate" you.
Clandestinely, perhaps slowly but it will happen. Prigs is a dead man walking, and he likely knows it.
The FSB is no different from its predecessors, the GRU, KGB, or the NKVD, in how meticulously ruthless, efficient and time-consuming they can be in searching, monitoring real, possible or imagined enemies and then carefully waiting for the moment to eliminate them. In the 1930's, an NKVD agent planted a bomb in a box of chocolates and then detonated it to kill a Russian dissident-in-exile, a NKVD agent used a ice pick-axe to blutchen Leon Trotsky to death in his Mexico City villa after failing a week before when Mexican Communists, dressed as bandits, started shooting rounds at his house from the streets outside. They even considered assassinating Hitler, in Munich, once in the mid/late 1930's by bombing his favorite Italian restaurant.