My Catfish had high liver enzymes at one point, but we changed food and meds etc and they went away. She also had partial seziures and some GI issues… it turned out she had Addisons disease probably caused by a pituitary tumor. Addisons is hard to diagnose because the symptoms can be vague and point to a number of things and it's also hard to test. If you don't test at the right time- during a "crisis" that's severe enough- you get a negative. We did more blood work on that dog in the short 3 1/2 years we had her then most dogs would have in their lives! I don't know that your boy has it though, this sounds gradual(?) and it usually strikes young to mid aged bitches.
I'm surprised you're vet hasn't wanted you to try anit-seizure meds- PB would be out because of the liver, but potassium bromide is liver safe. There are a lot of other less-used meds too. If they truly are seizures, the more he has, the more he'll get, and they could progress to grand-mals without treatment. I guess he could have a brain or spinal tumor, that's causing compression, but I would think he'd have other neuro symptoms by now.... How's his gait etc? any new limps, trouble getting up/off of things(not necessarily pain- even just seeming unsure). Have you done a tick panel? Lyme and other tick-born things can cause a myriad of symptoms.
Anyway- you might want to check out the GARD diet to help bring his liver #s down. Milk thistle/SAM-e are other things you should check out, it can do wonders. Catfish was on taurine daily, 500 mg, I think 250 mg is the dosage for a b sized dog but you can't give too much.
I hope you figure it out and have many good years left!