What should I do with my cat who keeps wanting to go outside?

I adopted a cat who is used to going outside and I let him go outside and he will come back in a couple of hours to eat. That is ok for me. However, I have a dog and another cat that stays inside and my outside cat brings in tons of fleas. The fleas affected all of my other animals and I don’t know what to do. I know how to get rid of the fleas, and I did put the nets on the windows back up. However now when I see my outside cat looking through the net, it breaks my heart because he looks so sad that he can’t go outside. He partially used to live outside and then come back and sleep in the house. I don’t know what to do. Should I let him go, or should I just teach him to stay at home? Is there a spray or something you can buy and put it on him so when he goes out the fleas’ aren’t attacking him and he doesn’t bring in fleas? I don’t think the cat leashes work? PLEASE HELP :(