Defamation suite against me by my son regarding a letter I sent

On 2020,my husband was ill,on knowing that my husband is ill my son threatened my husband and forced him and wrote a settlement deed in my grandsons name.One week later,my husband passed away.1 1/2 years later my daughter sent a notice to my grandsons about the settlement deed written by a illegal way.So they gave a plot and some amount to settle it off court and I, my son, my daughter,my grandsons all had a MOU written in front of the notary officer. My husband while writing the settlement deed wrote a life estate for the house that I am currently living I am staying downstairs and upstairs I have a tenant.My son has threatened the tenant to vacate stating that I can only stay but not earn through tenancy.So I wrote a letter to the university that my son is working about the illegal activities of my son.Now my son has sent a notice that he is going to file a defamation case against me.But i have no proof regarding my son threatening my husband.What should I do now