I just wondered where abouts on his body the tumor is located?
Hi Sorry it has taken so long to get back to you. I am sorry to hear about your old cat's tumor. It is very distressing when you can't cure a problem but I am sure you are doing the right thing not putting him through an operation.
Generally, I would always recommend using a low dose of antibiotics all the time or using a pulse-therapy eg dosing him 3 days out of 7. The Amoxy sounds fine but I would keep using it all the time to keep down the infection on the tumor because this (coupled with dead tissue/cells on the tumor) is what is causing the smell.
Also I would make every effort to keep the surface clean by clipping all the surrounding fur regularly to stop it touching the tumor surface. Make sure there is no discharge/blood from the tumor stuck on the fur. I would also use a very dilute solution of hibitane to flush the surface of the tumor twice daily. Also it may be possible to try and debride some of the dead tissue in the tumor by wiping the surface with a dry gauze swab. If this is painful for your cat or it causes excessive bleeding you won't be able to do this.
Applying the panalog or some other antibiotic ointment may help but it may clog up the area and add to the smell so use sparingly and only after flushing well.
I hope this helps you and your kitty.