Once you have finished the course you get the biggest feeling of satisfaction, because not many people would have a good enough bond with their horses to do it!
If you keep repeating this activity with him, and rewarding him, I would say it will last, horses love it when you treat them..(they're just like humans)! You could give him a chew toy hanging in his shelter, or a mineral lick? It obviously isn't a sign of aggression because if it was you would of already had a few chunks taken out of your leg, I would say it's from not having the correct start, so, you should continue educating him and make sure he has something to play with in his shelter and field (sometimes another horse is not always enough!)
I still haven't got to grips with this forum
so I'm not sure if I'am allowed to say this but...You as an owner and a rider have to think like a horse, it may help if you watch yours in the field for a few minutes...or watch some videos? I currently have 13 horses, 5 of which are under the age of 3, I have educated them to know that I'm the boss (I was not harsh or heavy handed in any way.) In the wild if a horse bites another horse, they will more than likely get a nip back, so that's what you should do (not necessarily nip them, because I'm sure you will end up with hair in your mouth and that's not pleasant) but maybe give them a tap on the shoulder whilst saying 'NO'. If it's not a nip, but he opens his mouth and acts like he is going to, you have to square up to him and say 'NO'. Just like when lunging a horse you should never really be infront of their shoulder, and some horses may see that as a sign of aggression so make sure your not infront of his shoulder!
All my horses go out for a hack around my fields with me, I ride my old show jump mare (in a headcollar and rug) and the rest follow us, I'm the leader and they know that being evasive or aggressive with me will not be tolerated and therefore with all of my own horses I can do quite literally anything, I have a broken four year old (she's only been under saddle for the past 4 months) and we are already jumping 3' fillers bitless, and I can take her on any road bitless, no one else could do that with her...You need to gain a horses trust.
I hope this is useful, it's quite hard to gage without seeing everything with my own eyes, maybe you could upload a video or something similar! E-mail me if you have any other queries!