I suspect that even though your SS hadn't seen his mum for 20 years, her death put an end to the idea that maybe they could be reconciled eventually. This can be hard for a person to accept. I agree that it's possible that your SS is still in the anger phase of grief, and he may have been looking for an outlet for that anger. Unfortunately, your DH was a convenient scapegoat.
Passports not in the drawer I always keep them in. Turning the place upside down.
Son’s girlfriend diagnosed with BPD