You could try The Blocker Tie Ring or The Clip Safe Tie. One of those might work well for you. I have never used them, so not sure if they would be suitable or not. I think that they would be a better alternative to tying to baling twine that would break instantly though, because the idea behind the blocker and the clip is that it resists a bit but gives a bit so it would give the horse more time to think and not just suddenly break free like with twine.
You could also try running a very long lead rope straight through a tie ring, don't tie it just hold on to it and groom or whatever and resist a little with it when he goes to pull back, if worse came to worse and you let go and he continued the lead rope would just slide through the ring, or try BlueSparks idea of looping around the post but not tying it. SmartTie Products Blocker Tie Ring Horse Safety Clip Equestrian Supplies Lead Rope Ropes - Blocker Ranch, Inc