My cat loves to go outside but I can't stop him from bringing fleas in the house. What can I use?

I have two cats. Only one crawls out my window that I cannot close. He climbs off the top of the roof to the ground everyday. He usually climbs back up every evening. So when he comes back up he is carrying fleas which jump on my other cat which eventually infest the apartment. I have fogged the apartment and the fleas are gone but I want to know what I can use to keep the fleas off my cat that goes outside so that he doesn't bring them back upstairs. But I want something that's not too expensive! what should I use? thanx Lindsey