how can a bank sue for mortgage payoff during probate?

I practice in Connecticut. You would be well advised to hire an attorney to help your son to settle the estate in this situation.

I am not admitted in Connecticut.  The attorney for the estate has to read the documents sent to your son and see what is going on.  Although I understand that your son is making the payments and the mortgage is current, two things come to mind: one that a mortgage is a loan made on the reputation, if you will, of the mortgagee.  Two, the bank may be trying to gain priority in the distribution of the assets of the estate by foreclosing.  Death of a mortgagee, who does not carry mortgage insurance, can accelerate a mortgage.  You need to read the documents.