My ex husband told our son he wasn't his dad: Advice?

I would like to ask other mamas how they would feel and act towards their ex husbands if their ex husband told 1 of yr children that they weren’t theirs ?? My ex husband told our now 25 year old son that he wasn’t his father !! My son come to me and asked me if he was and I said I swear on my mother’s grave that you are his and it has devastated our son , that his father could even say that !! Mind you he had affairs through out our marriage then ran off with his brothers wife at the end of our marriage !! When I knew my marriage was over, but we still lived together I had an affair but all my kids were teenagers!! Also I have a gut feeling that my ex has fathered 1 of his brothers children who is 16 now , as they come for a holiday to our place years ago I woke up and my then husband wasn’t in our bed so I went to see where he was he was in the lounge room asleep on the lounge and my ex sister in law was asleep on the matress beside the lounge and that didn’t sit right with me and my gut feeling was they had slept together ,a month after holidaying at our house she told us all she was pregnant !! They denied it and even to this day they do !! But this mumma saw red when my son come to me about what was said to him and I let rip !!! I demanded a DNA test for my ex , my son and my ex sister in laws daughter but they both refuse !! I’m all for it !! Thanks for reading my whinge !!