I’m not sure and I think there are people with more expertise who will come up with better solutions but I do feel like a tactical nuke is his trump card so I expect him to play it if things go worse for him given their comparatively low yield and very high shock value.
I think if a response was laid out to him in advance he might be less likely to cross that line. As it stands he can more or less choose to use a tactical nuke on Ukrainian soil knowing that much hand wringing will follow.
One example might be to say that Ukraine will be given a tactical nuke for use at their discretion on the battlefield for each one he uses. It would certainly cause pause for thought whether or not it was even followed through on.