Are you using blinkers? How often? Are you using them with all of your driving prep work? Is he confident in working with you when he can't see you?
When ours are ready to hook, the first thing we hook them to are Indian Shafts.
Basically they are just two shaft-sized trees, nailed together with a couple two by fours. We can't afford to go out and buy a new cart if one of them bails and tears it up. This inexpensive design can be torn up and it isn't going to be a b***h to replace.
You hook him with these just like you would a cart, and then long line him or lunge him. And if he bolts, hopefully after he is hooked, you just let him work it out for himself while he is working. Do not let go of your rope. Wear gloves, and lunge him or long line himlike it was any other day.
You might want to try a light sedative or even a calming agent for the first few times if he is working well unhooked and you are ready to hook him, and he is ready of course. I know many people are very much against it, but we have had great results.
When you go to pull the cart to him, are you coming OVER him, or are you pulling it up behind him? It is kind of hard with the Indian shafts, but it is not in your best interest to come down on him from the top. Just pull it straight in. Don't waste the time and energy by bringing it over him. Pull it straight in.
If he starts to get bully and tries to bust his way through, I'd honestly say put a stud chain over his nose and make him stand there until you have him securely hooked. Once he is, don't let go of that rope, but let him work it out. Ponies are tough little devils.