Thanks for using JustAnswer.com, I'm Dr. Dan and I'll try to help you.
If your cat went from normal this morning to very lethargic now I'd go to the vet as soon as you can. If your cat has been lethargic all day and has only vomited once or twice you probably can wait until tomorrow but I would recommend seeing a vet due to the recent weight loss. If his vomiting is getting more frequent then it is time to go to the vet now, especially if he seems like he can't urinate. I hope this helps, best wishes.