Try just cursing around the ring with him. Ask him to pit up a gait and then let him move around yhe ring where ever he wants as long as he's moving at that gait. Eventually you can start to ask him to go whwrestling to want him to once he has established a steady forward pace
I wish I had this problem with some of my horses...however, we do have one lazy TB. Some days he just doesn't want to work no matter how much you encourage him. But collection is the first step. When you get them engaged they have to use their muscles more and this will require some effort on their end. I warm him up with lots of bending and leg yielding like Sarah said. Switch up your routines so that he doesn't get bored keep his mind busy on what is next.
My gelding is the exact same way! What has been working for me has been lots of lateral work, shoulder and haunches in, and lots of leg yeilds. Also extended to collected transitions in every single gait helps a lot, and bringing a dressage whip to tap when he needs to speed up and isn't listening to your leg, it always wakes up my horse and gets him more responsive to the leg for the rest of the ride!