If those tests have been done recently then get him the copies of the tests (they are yours, you paid for them). Yes, they are expensive and if you don't have the money, you don't have the money - but sometimes they are cheaper than repeated trips to the vet. Keep giving the antibiotic - if he has an infection in his sinuses it could take the full dosage and maybe even more. Circulation and penetration for antibiotics is not all that good in the sinuses.
Care credit may be something for you to consider getting;
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Thank you for the good thoughts for Jasper. She has been ill for 21 years now.