I am so sorry. I know how frustrating pee problems are.
I'm not sure what (if any) additional information the vet may have given you, but it seems, based on what catloverami said, that this could be a congenital deformity that caused early arthritis? He's only 6, so pretty young for arthritis.
I wasn't familiar with phenylpropanolamine, which is apparently used to treat incontinence due to weakness of the urethral sphincter. Some meds are not as effective given as transdermals, so maybe you could try getting the pills compounded into magic chicken-flavored (or whatever flavor) "treats." That's what I did for one of my kitty's meds. Or you could try hiding the pills in a Pill Pocket, Temptations treat, or something similar.
Or...maybe his incontinence is due to something else, like a nerve issue? In my completely non-expert mind, it seems like this has to somehow be related to the arthritis in his spine and possibly a nerve being pinched or damaged. Maybe if you bring catloverami's info to the vet, it would give her some other avenues to explore?
The other thing that might be a possibility - but this would probably only be the case if he's also not using the box to poop in - is arthritis pain making the peeing and/or pooping position uncomfortable. A simple way to test that would be to give him pain meds for a few days to see if that helps. Buprenorphine is the best. It's a liquid that you would need to syringe into his mouth, but it's very quick and much easier than other meds.
Are you possibly close to a university with a veterinary program that might have a specialist you can see?