I need advice but I’m also just venting. 😩

My husband rescued a chihuahua/italian greyhound mix (we’ll call him Boo) a few months before we started dating. We’ve been together over 10 years now. Boo is now roughly 15 years old. He is going blind, mostly deaf and both of his front legs were broken as a pup before my husband rescued him. They never healed right and have been crooked for the majority of his life. Within the last year or so he has started really having issues with his legs. His joints will lock up and he has times of barely being able to walk, he has to drag one leg on the ground beside him. It’s heartbreaking. Along with his lack of mobility, all he wants to do is sleep. We close his crate at 8pm for the night and I open it again at 6:30 when I get up. I’m a SAHM so I leave his crate open the rest of the day after that but most days he refuses to come out to even go potty until 3-4pm. If I try to get him out before that, he just growls at me. When he does finally go out, he frequently pees outside, then immediately comes in and poops in the house and then goes back to his crate and usually only comes put for another 5-10 minutes the rest of the night to eat something.

He has zero quality of life and just seems miserable. I have tried many times to talk to my husband about it but he refuses to listen and will not let have a say or be part of the conversation. He refuses to take him to the vet and just tries to joke that he still has “plenty of life left in him.” It’s not funny and I’m at my wits end. Boo needs to be put down….out of compassion if nothing else. How do I get my husband to listen to reason??