Phenobarbital Side Effects

My 11-12 year old boxer had 4 grand mal seizures within 40 hours Tues-Thurs of this week. Bloodwork was fairly unremarkable- he’s been on thyroid meds for years and value came back at 2.0, 2.1 is low range for normal. Given his breed and age we are assuming a likely tumor cause for his seizures. We have been seizure free on Phenobarbital for about 48 hours (he had dose # 5 this morning).

I’m hoping someone can give some advice as far as what to expect to resolve side effect wise and what may not get better- I want to give him time to get used to the meds but his quality of life is extremely low right now.

We have the drunken gait/ataxia, face planting into the floor, furniture and falling backwards while going to the bathroom. We started circling/pacing the walls and occasional head pressing after seizure number 1 and this has continued. We are in cycles of 30min-2 hours of pacing, 90 min of sleep, then crying for 15-20 before pacing resumes.

He does not seem to know who I am/is not comforted by me when pacing. He has forgotten all of his commands and seems lost and confused with day to day routine. He does not want to do anything other than pace or sleep. I have not gotten a tail wag from him since we started the pheno.

How much of this might improve as he is on the med for longer? His quality of life was good for a senior dog prior to the seizures so I want to give him a chance to adjust, but am struggling with the fear that a lot of his symptoms are tumor related and that he is suffering for no reason.