Hi feegore, your poor little guy sounds like he is suffering from a combination of bad/poor diet and stress. Are you able to contact your ex-girlfriend to find out exactly how and what he was eating? That will really help diagnose/eliminate what is wrong. Does he have diarrhea at all? How old is William by the way? Is he drinking water?
I do not know of any drops to add to food or water to stimulate his appetite. You can try a fragrant herb like mint, it may help. Break it up so the smell entices him.
I do also agree with the vet visit, but only after you have found out what exactly was going on at the other place (give yourself a day or so to find out) because any vet is going to ask what you’ve been feeding him and if you don’t know, I am not sure a vet will be able to really diagnose what is going on unless there really is a parasite or overgrowth of some sort going on in the GI tract. You could potentially end up spending a lot of money on tests trying to diagnose things, so dig around, if you can’t find out then go straight to the vet and explain what is going on.