He's just absolutely awful. I don't give a flying duck if they weren't his dogs, that's his partner and child who will be beside themselves with grief, it's his job to be there. You sign up for this when you start a life with someone, I'm sure she's entertained plenty of his mundane tit that she doesn't care about. Difference is, this is the big stuff, and there should be no question of him being there or not - lets be real, this isn't an unavoidable one off, the fucker is never there.
Bin him Gemma, you deserve so much better, do you really want to face the prospect of 40+ years with someone who can't even show up for you when your dog is dying? You basically live alone anyway, why not give yourself a chance to find someone to actually spend it with instead of this little waster.