Westie Won't Eat

Hello everyone, I'm new here. I hope I'm in the right forum for this... I have a question about my mother's dog, a Westie about 2 years old.

Charley has always been picky about his food - my mother has gone through countless brands of dog food, even tried cat food. Most of it he wouldn't touch, and even the stuff he ate, he ate sparingly.

For about the last 4 months, he's lived with me, along with another dog and several cats. While here, he ate dog food (Dog Chow) with no hesitation and was much more active (not having a yard to run in at my mother's). When he went home last week, he dropped back into old habits and refuses to eat. He's also vomited once or twice since returning. He'll eat some scraps but not many (ate everything he could get here). He started losing weight while he was here, and we've treated him for worms and ear mites, but he hasn't gained any weight back. I'd really appreciate some advice on this, we're not sure what to do next.