My son lost his business and now has to file for bankruptcy. I am his mother and we own a lot that is paid for. Will we have to sell that?

When you say "we," do you mean your son and you? Then possibly...his interest in the lot will be an asset that will be taken into account in the bankruptcy. What will happen depends on many factors, including the type of bankruptcy, what he owes, his other income and assets, etc.

If your son does not have an ownership interest in the lot, then the fact that you are his mother does not matter--the lot is not an asset of his and will not be touched.