He sounds really unhappy. He needs to talk to someone he can trust, unfortunately he obviously feels he can't talk to you because he fears that whatever he tells you will be shared with your DP. Perhaps he has no reason to fear that, but he is only 10, and he can only see it from his POV.
10 is a really difficult age for boys. It is a time when they really need to be able to bond with their father, and it sounds as if your ds is feeling very insecure.
Does he look forward to, and enjoy his weekends with his dad?
Is there any chance that his dad could be encouraging his negative feelings about your DP?
Could he be feeling pulled in two directions?
It is possible that he is feeling left out. Of all your children he is probably at the most needy and vulnerable age.
Why is he so anxious about your DP knowing what he tells you? That would worry me a lot TBH.
Just my thoughts. I appreciate that I don't know you or your DP, so I could be off track.