Your pictures are beautiful, your baby is beautiful. Thank you for sharing. Mine was lost at almost 15 weeks and I wonder what he may have looked like, so it's nice for me to see maybe an idea of it.
As far as your ex and the girl. I think it's terrible what she's doing. If you can take something good out of it (like talking) then that's the best thing you can do.
As far as sharing with your other son? I'm not sure, I don't have other kids. But I do know that I will tell my future children about their big brother one day. These little ones that are lost so soon are not any less entitled to all the love of the world, which includes stories, memories and family.