There are numerous dog rescue organizations in your area, probably even one for the breed in particular. Since you are having all this medical work done, they should be glad to take the dog and find it a permanent home. Please look for a rescue organization to avoid putting him down. The vet may know of one. Good luck
I've already contacted Chihuahua rescue and other rescue orgs. He's a mixed breed, although definitely part Chi. So breed rescue is out. All of the reliable private shelters I've contacted are full. Spring is the most difficult time of year for them.