Yes, I agree with Anebel and Dantexeventer.
Right now it sounds like he's happy as a clam with no contact, but pick up the contact and he loses his impulsion as an act of defiance, thus becoming choppy and hollow. LEG! Push him up into your hands or else the work isn't going to help the muscles you want to improve, and ultimately you're going to have a hard time with other excerises as soon as you pick up contact.
While trotting have sufficent contact. Don't focus on him being supple until you've reached a steady rhythm (think of his haunches) in the working trot so that your horse is carrying himself. He'll probably be a challenge, and might even pull the reins from your hands, I'm guessing. But this is crucial: he must carry himself and drive into your contact so he's truly using all those muscles you want to target.
Do lots of circles, serpentines, etc and when you've reached the correct rhythm and impulsion ask him to be supple with your inside rein while keeping a nice contact with your outside rein.
Ugh, for a professional writer sometimes I have a hard time correctly expressing myself and hope that made sense.