I have a 3yr old filly that LOVES to nibble and chew on everything as well. She has destroyed 3 of my sweatshirts that I've had hanging on the fence, turned my back, then looked over and she has almost the entire sleeve in her mouth
. Anything that is dangly or new she must give a thorough inspection of first. I would suggest a toy too. You could get one of those Jolly Balls or Hanging Stall Toys. I've also heard horses like to play with Caution Cones and Yoga Balls (though I wouldn't put those in a stall).
Does your boy have a salt lick? I put a HUGE one in the coral and she loves it. She use to have a licking problem too, and now she doesn't lick so much. Also, I've set her up by hanging a towel, halter, and other various things she likes to chew on the fence. Then, when she approaches and tries to nibble or grab the item I give her a warning "Ah-Ah!". If she doesn't stop and still tries to grab it, I tap her on the chest or neck with a riding crop and repeat with an "Ah-Ah" or "No". When working with him, remember to have patience, this is a habit he has formed and it will be difficult to break. It will probably take quite a few sessions for him to get it. When you aren't around, I would suggest trying to keep everything he likes to nibble out of reach. If he chews on wood fencing or stall doors, they have special spray out there that tastes bad and is non-toxic to them but tastes bad too discourage them from chewing on the wood.
I haven't tried this, but if you got your boy a hanging stall toy after you reprimand him for trying to grab something off the fence, you could try showing him the toy. Once he starts showing interest or nibbling on the toy pet him and let him know that playing with his toys is okay, but chewing on halters, clothes and other things is NOT.