Keppra, Dilantin, and liver damage

For about 15 years my husband took 300-400MG of Dilantin and was seizure-free unless he went off of it, or took the generic. He is 34 years old and had seizures beginning when he was a baby. His current neurologist had frequently wanted him to go off of Dilantin because of the long-term side effects, but let him stay on it as long as all blood tests were ok. He hasn't had a seizure in over 10 years. About 6 months ago, his blood test showed elevated ammonia levels, so his doctor started him on 750mg of Keppra 2x per day, and slightly lessened his Dilantin from 360mg to 300mg. The next blood test showed normal ammonia levels. The Keppra made him very tired at first, but since then the only side effect has been some excessive irritability. Still no seizures, but we don't know if that is due to Keppra or Dilantin. Now his most recent blood test showed elevated ammonia levels again, so the neuro wants to increase his Keppra to 750mg 3x per day, and after a week, drop his Dilantin to 200mg. He hasn't been on that low of a Dilantin level ever. We are super scared of the side effects of all this. He drives and I am due with our second child in April. The doctor says it has to be the Dilantin causing the liver damage because Keppra doesn't get processed through the liver. My husband doesn't drink enough water (he sometimes goes hours and hours without peeing during the day)...could that cause elevated ammonia levels? Why is the doctor dropping down the Dilantin so quickly? He had previously told my husband that he would do it gradually. We are seeking out a second opinion, but does anyone else have experience with this type of situation? Is there any other possible explanation for ammonia levels that go up and down?