Personally, I would take him and a stool sample to the vets. When a cat is dewormed, there are many choices of medication and some just don't kill ALL the possibilities. He may need a different med.

If he isn't eating much try something more tempting like canned food. There is always the possibility he is a fussy eater. Also, at 7 years of age, I would have his teeth checked.

It could be nerves, new food, parasites etc, but why wait 10 days if there is something that can be done now?