They are not unpredictable at all, they are just bored out of their minds and are trying to entertain themselves. They have got away with the little things and these are escalating to become major. He has to be of a fairly sound temperament to not have turned nasty
If the conditions are as you say then it is a case of mental cruelty.
Before I would do any sort of work with this horse I would have him turned away for a couple of weeks, out at grass where he could kick up his heels and get rid of the energy and just relax and be a horse.
I would then start to work him but not in an arena by taking him out on the trails and getting plenty of long rides into him. Lots of slow trotting not asking him to be on the bridle but allowing him to enjoy being ridden.
His mental welfare needs to be looked at first. At the same time he b=needs to be kept under control when on the ground and for that I would use a chain across his nose.