Flea treatment for cats with leukemia?

My cat has had leukemia for the past 2 years and hasn't had much trouble with fleas. Whenever we use flea killer on him, it makes my cat sick, more or less, it just worsens the problem for my kitty. I would like to know a brand name, or product name for something that will kill my cats fleas and leave my cat unharmed. Thank you