The weight loss can be a few things-
1. Parvo virus- less likely in a vaccinated adult and you should be seeing diarrhea
2. Intestinal parasites (as in worms)- your vet would have performed a fecal test (evaluated the poop) and given you de-wormer at the visit
3. Mange- if he is itching excessively, they will eat less and lose weight with that alone. Mange is typically treated most effectively with oral medication (either Ivermectin or the new oral flea products like Nexgard or Bravecto).
4. Gastroenteritis- anytime they have an upset stomach (which would cause the vomiting), they will eat less and drop weight. You can start with over the counter Pepcid AC (famotidine) giving 10 mg by mouth every 12 hours to reduce stomach acid.
If you just saw your vet, I would call tomorrow to discuss the weight loss and not eating. If you haven't, I'd recommend a hands on physical exam in the next few days to determine the cause. I'm sorry that those symptoms can be linked with so many different causes.
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