It's like you have to spoon-feed him things-----making sure the good stuff is the stuff that's visible. At first, one has to be smart when doing this.
But, many times, when this positive reinforcement is used, you can insert all the lessons which he might not want to learn without the positive reinforcement (the tractor).
Think of: Ice cream and baby aspirin. Mothers and fathers do this all the time. They mix the medicine with the ice cream. Very smart thing to do. The kid likes it, and he/she doesn't know he/she is taking medicine, too.