Is he windsucking or cribbing? They are two different things. Cribbing is when they bite on something and gulp down air. Windsucking is where they will do the same thing without having to use their teeth to grip. My horse developed crib biting when his routine was changed from 24/7 turnout to being stabled at night. Can only imagine he got stressed/bored at being kept in and not being able to graze constantly. Obviously, if you can turn out then this will help but if you have to rest him then provide shed loads of hay for him to nibble on. Remove anything from the stable that he can get his teeth on. Its thought horses that crib have excess acid in their guts so it might be a good idea to have a vet check him out as he could be developing ulcers.
Unfortunately, once this habit has developed it is nigh on impossible to stop. However, don't stress yourself out about it - you just have to learn to live with it - plenty of owners do. My horse cribbed all his life and lived to 30 happily. He was a brilliant all-rounder and I would dearly love to have him here with me again.