Well I don't know much about the story or your ex but that sounds fine to me.
It's polite, straight to the point and isn't intrusive.
If he takes offense and gets all wierd about it then he's immature.
You could also word it as such:
"I'm making a scrapbook for bubs and I'd like your family to be included. I want to ask your mom for some photos of the family I can have. Just running this by you. Is this ok?"
Same thing really.. Just different wording
