Next I would put this boy to *work*. Start on the ground with lunging using constant direction changes, backing, moving all body parts....make him move, get him really listening to you, wondering what you are going to ask next. Praise him when he does something right, but quickly move from one thing to another to keep him guessing.Clinton Anderson's lunging for respect 1 & 2, very helpful stuff. Also his sending excercises, getting the horse to go where you want him to with escalating aids until he responds, really gets the horse focused on you.
On his back I would initially do some giving of the five body parts to make sure he is responding and safe, and then *work*. Lots of trotting at first, smaller circles, frequent direction changes, frequent gait tranistions. Stop, back up, trot off, I mean *move his feet*. He should be so concerned about what you are going to ask of him next he has no time for naughtiness. Then move on to cantering circles, if he throws in a buck drive him foreward, hard, get him tired. Also with cantering, I like to do a lot of sharp stops, backing, immediate canter departure. Gets the horse working off his hind end and makes him pay attention.
Most horses like this, you can gradually decrease the intensity of the work and make the schooling a little less intense. They start to realize that if they behave, life is easy, if not, they get put to work. Some though, will need constant work to do to keep them occupied. I don't like those guys, they exhaust me their brains just never seem to settle. Being the cobby sort, I suspect your fellow is just testing the waters and getting into mischeif without enough work to do. Most horses need to be put to work when they start to act up every so often, my own mare no exception. Especially when they are first getting to know you, they are testing you. Let him know that if he has the energy to act up, he has the energy to do some hard work for you. He should quickly gain respect for you.