Instead of a posting trot/extended trot, try a very slow trot.. almost a jog. Just little changes like this can cause a horse to slim down due to working more as it's harder for a horse to go at the trot slow and balanced, than big and fast.
Make sure his diet isn't too fatty.. some store-bought grains are so rich that amount of grain they need for energy is piling on a MOUNTAIN of fat, thus causing a horse to become obese and sometimes unruly.
Also, lots and lots and lots of transitions. Lunging 4 laps walk, 8 trot, 2 walk, 4 trot, 3 canter, 2 trot... it's a lot of work. Just keep in mind you don't want to kill your horse :P Just make him think and put effort into it.
Also, any sort of hill work is awesome. It not only builds up muscles, but burns off fat! And it's great for you too :)
Constant work isn't good for them if they are obese. Keep it moderately short and give them some time to recoup as well!