Just curious to know what type of set up is he kept in? Is he stalled or pastured?
A young stallion just like any young horse needs freedom from work. Does he pin his ears when he's charging? I've known several horses that resort to this type of behavior b/c of lack of open space to run & play and lacking horse companionship to teach them that they are still a horse. Its hard with a stallion b/c obviously he may show aggression to geldings and you may not want him breeding I've owned and had the pleasure to work with many stallions and when they are young they are very good at testing the waters, especialy when they've been raised by others that may have been less firm. Be confindent, truley stallions are so fun once they are trained, I personally prefer a Stallion over anything, but I love a good challenge
Good Luck and Stay Safe