Wave your arms, whistle, make a lot of noise to try to spook your horse. He will pull back, but he can run out the length of line. Pull it short again, and repeat, and repeat, and repeat. He won't feel trapped, and he'll learn that being tied isn't scary. It's going to take a LOT of sessions.
When I was working with my older gelding (he was a 15 yr old OTTB, long off the track, but still had issues with being tied) it took me two or three sessions to be able to tie him properly. Same with my little mustang mare. The gelding I am about to buy has issues with being sprayed with fly spray and will pull when I try to spray him, but the underlying issue is the fly spray, not the fact he's tied.
Hope that makes sense...