Suggest Remedy To Improve Phenytoin Level

hello, my father has recently undergone brain tumour surgery as he just suffered parelletic attack. He's taking the medicine to improve the phenytoin. Last month after taking 400 mg everyday his phenytoin level was incresing @ of 3.2mg/dl. Bt now when we tested after a month the phenytoin level has again fallen to 5.64 which was earlier improved to 11.64. suggest me something?