Do you think all the radiation just sits there? Say he does a 10kt ground burst to really get the radiation to be a problem. What is to stop the winds carrying it over his own troops? Or into Russia or Crimea? Or into Europe?
Japan blasts were 12 and 21 kts but much dirtier than modern weapons.
He gives up the land. You can always retake land, you can't put a nuke back in the bottle after its gone off.
Nukes are much cleaner these days, I don't think it's that much of an issue