Stable chewing.

[poiuytrewq]

One of our horses has suddenly a few weeks ago started destroying his wooden stable?
He's 6/7 and we have had him since 3. He's always been great in the stable, stands quietly- even alone or waiting for hay/feed.
This year we made enough hay to last the three of them ad- lib over winter (a huge weight off my mind). The others eat it fine but this one dislikes it for some annoying reason!
I tried at first ignoring it with the theory that if the others ate it then it's fine and he's just being fussy as it's new.
Then I noticed the damage to the stable begin so I did go and buy him different hay, which he eats, not a huge amount but he is eating it but he's still gnawing in the stable!
How can I stop this before he becomes a horrible destructive little horse!
I'm going to pick up some crib box but any other suggestions would be helpful.
Does it show a deficiency?

 

[AdorableAlice]

If it is a sudden change in demeanour I would be thinking something is wrong. Tooth/mouth pain, ulcers ?

 

[FairyLights]

Can br due to lack of salt and often happens in January and February for some reason. Add up to 2 heaped tablespoons of salt into the feed. Start with a small amount and build up slowly. make sure there is plenty of water. 2 of mine chew wood at this time of year inc the post and rail fencing. i have found this system stops or greatly reduces the chewing. they have salt licks but mustnt use them enough.