In '96 when I lost my hound Annie it was very hard on me and my Chihuahua LaVie. LaVie was very depressed.
To get her to eat I had to hand feed her at first, things like chicken, tortillas, and a bit of mac-n-cheese. Sounds weird but these were her favorites and it was all I could get her to eat at first. In time she went to more normal foods and then back to her food.
I sat with LaVie for long periods just talking with her about how much I too missed Annie. You see, Annie was one of those rare "soul" dogs that you bond with beyond being able to explain.
LaVie came through it okay but she never did more then tollerate other dogs after that. She didn't play with any other dogs again, just humans and an occasional toy. Not even my parents or gramma's Chi's. LaVie lived a long life, I lost her in '04, she was almost 16 yrs old. So, it changed her but she was okay in time.
Good luck with you boy. Please keep us updated.
This is an old picture of LaVie and Annie, from '91 I think. LaVie would have been about 3 and Annie about 8.

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tabitha
Kayo, Kahla, and Dante'