cricketblad, I am sorry you didn't get an answer before.
I'm also sorry but I have no experience with phenobarbital treatment as I never had a dog with those issues.
It is hard to tell if his recent symptoms are as a result of his original condition....or side effects. I think you need to have a good discussion with your vet about this.
In the meantime, at least your question has been 'bumped' so it will be seen a bit better, and I hope you will get a reply from someone else who has more experience.