Wow, what a bully.
Clearly you are a better woman than me. I would have gone in the kitchen, gotten a wooden spoon, come back out, smacked him in the head and said "Excuse me. Mommy is talking. Next time you interrupt me I'll get the frying pan."
Well I clearly have a problem with detaching so maybe I would just say- "I'm sorry my story is are bothering you. Maybe the lawnmower will drown out the details while you are mowing the lawn. Didn't you tell me you were working in the yard ALL DAY for me for Mother''s Day? I love you sweetheart."
Clearly I have a lot of work to do. Yeah, I do much better with no contact.
Sorry you are having to deal with Debbie Downer on Mother's Day. Maybe you can get out of the house for a while with your son and do something fun and special just for you.