AITA for suggesting my brother find someone more compatible instead of pushing his son to accept another mom?

My brother (30s) is a widower with a 9 year old son. His wife died four years ago and for two years he has been in a new relationship which has recently hit a rocky patch. My nephew gets along well with his dad's girlfriend and they had a sweet relationship. But recently there was a talk where my brother and his girlfriend sat my nephew down and asked him questions about them getting married and her being his bonus mom. My nephew liked the idea of them getting married and was smiling until the mom talk began. He said he doesn't want another mom and he asked why he needed one and he became upset. There were more questions asked but it was clear to my brother and his girlfriend that my nephew was not open to it at all.

She doesn't want to not be accepted as a second mom. She doesn't mind talk of my brother's late wife and such, but she said she refuses to raise children who do not see her as at least one of their moms or parents. My brother's upset because he loves his girlfriend and he can see a future with her. She does love him. But not enough to want to be the other in a blended family. That's how she put it to my brother. He's fighting for the relationship and has talked to my nephew more and has asked him questions about why he wouldn't be okay with it and trying to press my nephew to be open to the idea that he could have more than one mom. He consulted a therapist about it too.

He vented to me the other night over dinner. I heard him out and let him talk about how desperate he felt so save this relationship. He said he didn't want to be single for another decade because of this and he doesn't know if anyone would want to be with a man with a kid who's kid would never let them become a true parent to them. I suggested he could find someone compatible, who was cool being liked and even loved one day but not a parent. I said not everyone expects that from their partners kids and that his son would like it more than this pushing he's currently doing. He told me it won't save his relationship with his girlfriend and they're compatible in every other way. He said he can't lose the woman he loves. He didn't think he could fall in love after his wife died and he did and he can't stand the idea that it'll end and maybe he won't find anyone else. I tried to comfort him but my suggestion of finding someone more compatible angered him to the point he asked me to leave.

AITA?