Best possible way!!!! Find a dog that hates cats ^^ and borrow it from it's owner for a few weeks. Keep it on leash most of the time... just let it free sometimes (under your strict supervision!) in the garden. This will add enough "psychological" (^^ LOL ^^) stress on your cat to make it shed the extra fat....... http://www.dailymotion.com/video/x2rndy_tom-and-jerry-dog-trouble_shortfilms

Make sure he is not eating any people food. If he is not, then give him a bit less cat food and play some games with him to make him more active. But if he is an older cat, I think it is normal that he is a bit fat. ^^'

take him to the vet

Eat him!!! This is a joke... ;)

Play games with him! Or give him something to scratch/play with. Good luck ^^