I'm just going to throw my 2 cents in on the BITING (nipping is the same thing as it's disrespect). First, I won't hit a horse on the head because horses don't bite each other there. A horse that bites another is charged at and gets bitten usually on the neck. If the instigator is quick enough at ducking away, it will get bitten on the back or rump. But, rather than start hitting on a horse, try flapping your elbow closest to his mouth. Do it randomly. Your elbow is sharp and he may turn into it once but he won't want to a second time. The randomness keeps him on the alert. PS don't let the "early stages of colic" allow you to dismiss his behaviour as being caused by this diagnosis. Early stages means hours, not weeks or months.