Now that he's broken off 4 tmes, you have made it MUCH harder for yourself to train him not to pull back.
I am a firm believer that every horse should be taught to hard tie, but would not recomend that you do this yourself if you are not overly experienced in the field.
For now, I would work solidly on teaching him to give to pressure on his head. Put a rope halter on him (the webbing ones are useless for teaching a horse to give to pressure) and work first on getting him to drop his head when you put light downwards pressure on the halter. From this, get him stepping forward when you put some forwards pressure on the halter, ensuring that you remove the pressure immediately when he steps forward. He will learn that bracing against the pressure will make him more uncomfortable, but giving to the pressure by stepping forward, will relieve the pressure and he can be comfortable again.
When you can get him working well off pressure doing groundwork, run the lead rope through a tie up ring (long lead will be easiest for you) while you brush him. If he stands quietly, leave a loop in the lead. If he goes to pull back, keep the tension on the lead until he gives, then leave a loop again. He'll soon learn the pulling back will not help him and he might as well stand quiet and comfortable.
When he has mastered this, I would be looking at teaching him to hard tie. Please get a trainer to do this for you if you haven't done it before.
Basically, the horse is tied to a fixed object by a lead rope on a rope halter, with a neck rope also tied to the object to avoid inury to the poll.
The inner tube of a tire around a tree branch or cemented post is a great tool to tie to as it has some give, but won't break.
Once tied, the trainer walks away and leaves the horse tied here. The horse will pull back and fight the pressure, but will work out pretty quickly that pulling back is useless, and will stand quietly. This is great to teach the horse that even if there is no one around, it must stand patiently tied.
Good luck :)