desperately need help/advice on dieting/exercise in cats.

so this is tornado, hes my soulmate and i love him beyond expression. he weighs in at about 25 pounds (he was a normal sized cat before being neutered, idk if that causes weight gain but thats just the time line).

i also have 3 other cats, they are all a pretty healthy weight, one of thems slightly chubby, but nowhere near as big as tornado. i want him to lose weight and be more healthy as i want as much time with him as possible, but im not sure where to start. having 4 cats, the most convenient way to feed them for me and my family’s lifestyle is to free feed them, i wouldnt even know how else to feed them so advice on that would be appreciated.

im getting worried about him and want him to maybe lose a little weight because all he pretty much does is cuddle with me, eat, sleep, and poop. when i try to play with him it doesnt really work because he just ends up laying down and just using his arms, which i cant imagine being majorly beneficial to his health.

i really dont know how to help him or where to start. id imagine it would be easier if he was an only cat, but he is not, and there is really no keeping him separate from the other 3 because i live in a condo.

any advice or help will be beyond appreciated, and i am able to answer any other questions about him.