Male cat bladder infection

My cat, Joey, was diagnosed with a bladder infection 4 days ago. The vet prescribed Clavamox 125mg twice a day. (Joey is 14 years old and weighs 17 lbs.)

The reason I became concerned about Joey was because he started peeing outside of the litterbox on the mat right next to the litterbox (but he still pooped IN the box). But after 4 days and 8 doses of Clavamox, he is still peeing on the mat instead of in the box.

How long should it take for the antibiotic to work? Should I be concerned that he's still not peeing in the box? 

We do have a follow-up visit with the vet in about 10 days.

Thank you for any help you can provide!   

When a cat has a uti it hurts to pee. They associate the pain with the box and pee elsewhere in the hope that it won't hurt if they go elsewhere. It takes a short time to create a negative association and a long time to undo it. Different type of box, different position, different litter...any or all of these may help him break the association. Him still peeing outside the box doesn't mean that the drugs aren't working. Hope he's better soon.