I have a 3 mth kitten with a sinus infection. Vet put him on 10 days of Clavimox its not working. What will?

I have a 3 month foster old kitten with a left over sinus infection. He did have pnemonia in the left lung but it cleared up with Clavimox. He still has trouble breathing, has thick mucous draining from nose. Left eye has some drainage. The vet put him on Clavimox again but it is not working for the sinus isuse. He eats very well, plays all the time and is very loving. What antibiotic would work best for the sinus infection? He is soooooooooo ready for his forever home but not with the sinus problem. Thank You, FosterMom Cindy