What's wrong with my dog?

My dog is an 11 year old German Pointer. We are currently fostering him because he has been very depressed in the shelter, and were going to adopt him after he got spayed/neutered (idk what one is for boys). He was basically never eating for the whole month and a half he was in the shelter, and his ribs are showing a lot. When we brought him home, he would eat food we'd give to him. Although he didn't usually eat all of it, he still ate some. He goes to the bathroom every few hours and this was expected 'cause he's old. However, starting maybe 2 days ago, he hasn't been eating and using the bathroom CONSTANTLY. We'd take him outside, and 5 minutes after coming back inside he'll ask to go outside again. He's not just asking just so he could be outside either, he's actually using the bathroom every time. Yesterday morning he threw up also but that was the only time. He seems to be more out of energy too. We are going to bring him to the vet if this continues, but my family is upset out it and I want at least a vague answer