Lots of time, it can be a nutrient definecy. Is he wind sucking? WHen he pulls back is he sucking in wind? If not, he maybe wood chewing. That is a little easier to stop. Farnman's Quitt is supposed to work and if not, money back guarentee.
If this horse is anything like my friends then soap will not stop it. It can stop them from doing it on certain objects, but if they really want to do it they will find something to crib on. My friends horse just moved on to stumps, the water container and she will even crib off her pasture mate! All she needs to do it is something to put her mouth on. The only thing that may work is a cribbing collar.
If it is cribbing, which is wind sucking, then the weaver miracle collar works wonders!
If it is wood chewing, then quitt helps to slow it down. When I used quitt it helped them to stop chewing in the stalls, but if I tied them somewhere they were right back at chewing at the post they were tied to. If it is just pure bordem causing the chewing, I would find him something to do to take up the excess time. Maybe extra hay, a toy, more workouts...etc
The difficult thing about cribbing is most of the time you can't make them stop especially if they have been doing it for a while. My mare I used to lease cribbed and the only way we could prevent her from cribbing is from a cribbing collar. You could also try things like putting hot sauce on everything he can crib on, but cribbing collars work the best in my opinion.