Dr. wants to stop Dilantin due to liver issues

My husband has been on Dilantin for about 20 years now. Once he got on the brand instead of the generic, he has been seizure-free for the past decade. In fact, I have never seen him have a seizure. He was on 300mg of Dilantin, then 330, and 360 for the past year. His recent blood tests showed his Dilantin level is too low, and his ammonia level is too high. His doctor says this indicates the medication is causing problems with his liver. He also has issues with his gums and sometimes his short-term memory. However, he is very happy he hasn't had any seizures. He even drives now and has a full-time job. Now the doctor has him starting Keppra. The initial prescription is 750mg twice a day. He took his first dose (generic) was last night, and so far he has slept almost 12 hours, which is very unusual for him. There is no way he will be able to work if he is tired like this. He is very afraid of weaning off the dilantin...the doctor said he will start that when his Keppra levels are high enough. Also, we are concerned about generic vs brand. Generic Dilantin didn't prevent his seizures. The brand-name was covered by our insurance, but brand-name Keppra is not. It would cost us $300+ per month for the brand. Is there anything we can do about the sleepiness on Keppra? Will it go away? Is it possible to just lower his Dilantin a bit and see if it still prevents his seizures even at a low level...instead of doing the Keppra? What about a low level of Dilantin combined with a ketogenic diet? We are just trying to figure out an alternative to the Keppra because we really don't want to start over with medication trial-and-error at this point. Dilantin has been working for so many years...