First of all, he has to have an impulsion to ride from behind and come on bit. That's what I started to work with mine with my new (that time) trainer: straightness and impulsion (heck, we lacked everything!
). Ask him to move forward with your leg/seat, if he doesn't - touch him with the stick. Then catch it on front (so he wouldn't go fast, but would go round). It takes sometime to build muscles, but correct riding WILL build them.
You can also introduce ground poles and cavaletties (on low position) into your work - those help quite a lot. As well as up-hill trotting.