I’ve had this. My DSDs have said that I’m a hypocrite, ‘Mum says so,’ or that I have not right to... mum says... list anything you like!
I wish I’d challenged that more, I used to be caught off guard and I didn’t want to put down their mother. However now I think it’s a good opportunity to challenge. And at least it’s external, your DS isn’t just thinking it, that shows a level of innocence and trust in you.
Say ‘oh really, no I don’t think he is/ I am... because... ‘ and then say people can have different opinions, and that sometimes people say things when they are upset or angry. That he has his own opinions and never to feel that he can’t tell you things.
I wouldn’t so much ask for his opinion, it can be putting a kid under the spot, but leave it open for him to say more, encourage a relaxed natural conversation.
Eventually I did start defending myself, to my DSDs, who also said bad things about their dad ‘mum Says all men are good for is money’ and ‘mum says all women are bitches’. We’d then say, oh I disagree!