Clavamox is very tough for some cats to tolerate. It may be the clavamox, but I'm not entirely confident with symptoms developing a week later; you probably would have seen the diarrhea and vomiting start while he was on the meds.
I highly suggest the vet at this point. With vomiting and (bloody) diarrhea there is a risk of dehydration, lack of food for a prolonged period can affect the liver and, as mentioned, you have to be careful of diabetic complications. If it's a GI infection and an antibiotic is needed, he'll probably go on metronidazole.
IMHO, I would watch him carefully overnight and get him to the vet first thing in AM. Any liquids he will tolerate will help. Don't wait much longer.