What is wrong with our bichon mix ?

Donnie is eleven years old. Two days ago he stopped moving around . Yesterday, I awoke to a 7” diameter pile of vomit and a little bit of feces on the tiles. He kept to himself and , his previously heavy intake of water almost stopped. Today we hoped he’d be better but he won’t eat and is hiding in the bushes. If we coax him inside he manages to go back out and hide. He drank a lot of water an hour ago. There is no response when petted nor any when our other dog sees someone and starts barking. He just sits there and ignores her. There is some vomit on the pillow he slept on last night. When he walks, which is rare, it’s super slowly. Are vets able to diagnose this illness? He also has cataracts and runs into things.
tia