(You hang it somewhere close to the manger)
You can find lots of DIY toys for horses on Internet. It's not exactly a chew toy, but one I found quite cool is tying bits of apples and carrots along a thread so that the horse tries to eat it and doesn't get bored.
Jolly Ball!! They are so effective. You can also try a mineral lick or fill a milk jug with some carrot pieces, hang it up in the stall and he can play with it
I would say a jolly ball :) or if you have a regular ball you could use that :)
I would suggest a jollyball or put a salt lick near his manger so he'll chew and lick on that instead.