It sounds like your little man needs an attitude adjustment and a lesson in minding his manners. As far as the nipping, I personally use a verbal command immediately after the offense. Horses only have a few seconds of relay process before their brain is on to the next thing, it is imperative that you scold him for the behavior immediately after he does it. If a loud "no" doesn't grab his attention, a tap on the nose may be necessary. Be advised that too excessive force could lead him to become head shy or skittish if you over do it. You don't have to flip out on him, just scold him, as his dam would if he nipped her. It sounds to me as if his lunging and charging at things he needs to be reminded of how to lead properly and respect space. When he begins to pull and get ahead of you, as if preparing to charge, try stepping in front of him to block him and give a verbal cue like "whoa" or "easy" and if he still gets anxious, you may have to gently swivel the lead to encourage him to back up. With horses that try running me over I like to teach them to respect space by swiveling the lead or snaking it like you're playing with one of those ribbon dancer tools that were popular in the 90's. Some horses really respond and others just try to jump or run around me, in that case I try a lead shank. Some horses simply comply just with the lead rope over their nose but others need a chain. Be sure to only apply pressure when he begins to charge and immediately release when he gives to the pressure. Keep working with him every day, keep his sessions short and always end on a positive note. Most importantly you need to remain consistent so he gets the idea and begins to behave. You may want to avoid feeding treats by hand to discourage nipping. You also want to be sure that every time you lead him, you reprimand him if he lunges or charges so maybe for now you should avoid letting children or inexperienced handlers lead him until you straighten him out. That way his training remains consistent and nobody gets hurt. Personally, I would avoid riding him until you straighten out his ground manners. It sounds as if he may be reacting to too much weight or perhaps he is not really "broke" to ride, just because someone sat on his back does not mean he is broke. I do not ever recommend kicking the sides of a horse while riding (except if the horse is attempting to lay down and roll on you) many horses buck in response to being kicked, gently "squeezing" to apply pressure is the appropriate cue to ask a horse to move forward.
