Exactly same thing happened to a good friend of mine. Friend was single mum for yrs with DS.
Met DP, DP ended up moving in, friend got pregnant, DS hated partner, DP 'lorded' it over DS. Resentment grew between the two of them, DS blamed DP for everything that went wrong/missing etc, DP complained that DS was controlling friend and making everyone unhappy, DS refused to speak to or do anything when DP asked.
Eventually, it all came to a head, DS moved out (with no warning when friend and DP were out shopping one day) to live with his GP's, who were more than happy to take DS on, friend was devastated, totally in bits, begged DS to move back home, DS refused.
Friend eventually realised that DP was a total cocklodger control freak and an arse, with serious mental health issues, and their relationship broke down. Friend is now single mother to a toddler, while her DS still refuses to move back home.
Friend says her relationship with her DS has changed since he moved out and the way he did it. She now regrets listening to the DP. She wishes her DS was home and everything was back to the way it was before DP came along, but she says it is gone, forever now.
DS is angry that friend never listened to him, that DP was controlling and devious. DS says he feels friend chose to be with DP over what her DS wanted. That made him feel unwanted and pushed out.
All this makes me wonder what yr DS says his problem with your DP actually is. Do you think your DP is being as fair as possible?
I would hate another family to go through what my friend has been through. She fell apart when her DS left home. She is still not over it now. She cannot talk about it without tears in her eyes. It all came to a head almost 2 years ago now, but she is still ripped apart by it, because she says she lost her DS for the sake of her DP.