I'd ask the vet to do an X-ray or barium X-ray. He could have an obstruction in his stomach or intestines. My friend's cat once ate a piece of surgical tubing, and it went down his tubes but caused the kind of symptoms you are describing.
Hey there and welcome to the forum...
Marty absolutely needs to be seen by your vet ASAP. All we can do is *guess* what's wrong. This could be kidney disease, liver disease or hyperthyroidism. This could also be something minor.
It's great you're assist feeding. Very important. You don't want Marty going more than 48 hrs w/o food or water, as this can develop into Fatty Liver quickly.
I implore you to make an appt to see your vet tomorrow.
Please keep us updated. We care!