This sounds a lot like my story

I'm not really comfortable sharing details in the open, but if you want to PM me, I'd be happy to share/commiserate

I will say, it's a very difficult thing to watch, when you're children are ignored and his other child is doted on.
In your situation, I think I might discuss the issue with his mom (your daughters' grandmother), assuming you have enough of a relationship with her where you would be comfortable doing so? I can't imagine it's easy or sits well with her, if she wants a relationship with your daughters, to have to have separate lives with her grandson and her granddaughters. She may be willing to facilitate something, or she may give you some insight on the little boy's mom and how she would feel and maybe help the two of you connect?