Took Popeye To Vet

I took Popeye back to vet because I don't think he is eating enough and everytime I give him his medication he throws up mucus. The vet told me to finish the medication she gave him last week then start Clavamox for his URI. She also said his fever broke and his congestion was improving. HE DID NOT LOSE ANY WEIGHT. So I was told not to worry about how much he eats he is maintaing his weight at 11 lbs. 11 oz. Can anyone suggest anything else to help him get better? I have kept him away from my other cat as much as possible but once in awhile peanut sneaks in the bathroom . By the way my vet only charged me for the medication because she would rather have me get him healthy then return him to the SPCA. I'm wondering if maybe a different food might help any suggestions. I finally have him eating 1/4 can of food and he eats meow mix, eukenuba, and some cheap stuff that is a store brand that as soon as it is gone will not be bought again. Sorry so long but I am still worried that we are missing something.
Thank-you in advance for your support.

You can try heating it up for a few seconds in the microwave. Cats eat mostly by smell and when they have the sniffles it's hard for them to smell it. Heating it up brings out the smell. Also try putting some chicken broth or tuna juice on top. Might help make it seem tastier.
He's probably throwing up because he doesn't have enough in his belly before he gets the meds. Try heating it up and then give him the meds. It will help with his upset stomach.