This is the best summary of why he won't go doomsday in my opinion.
"Defeating the Ukrainian army on the battlefield would require dozens of tactical nuclear weapons to achieve a decisive military effect – not just a single strike. In fact, the US Army planned to use 136 nuclear strikes to achieve a decisive result over a 60-mile-long front – and the frontlines between Ukraine and Russia now stretch for almost 400 miles. The risks of radiation from this many strikes, not to mention
the psychological effects on Russian soldiers of witnessing destruction on such a scale, would be considerable, reducing their combat effectiveness and their already-low ability to exploit gaps in Ukrainian defensive lines. The radiation effects are not to be ignored, with immediate effects on all soldiers in the open regardless of their loyalty, as well as the spread of fallout into occupied Ukraine, and into Belarus and Russia, which would cause panic among populations already made wary by Ukraine’s strikes in Russia and occupied Crimea. Further, if the intent is to break Ukraine’s will or the West’s resolve, any such strikes would fail utterly. Breaking the 77-year taboo on nuclear use in war would only galvanise Ukraine and the West’s support for it while rallying more countries to the cause."