Assuming that the horse is broke to lead and you can handle his feet my advice is to leave it alone. Your trainer will probably thank you. Overhandling a horse does not make them easier to train. When I get a young horse in to start, I would like the horse to lead and not be terrified of me. If I can get shoes on it then that's even better. As a trainer I want to desensitize the horse as I train because I might need some of that sensitivity to teach something then I will desensitize later.
ALL HERE WITH THIS. (I had written this nice response.. and then my computer crashed during editting!
Okay, so IMO, Kevinshorses is completely right.
Keep this in mind: one "horse" year is the equivalent to three "human" years for mentality.
Even where my yearling is, there's an older filly of 2. And let me tell you.. night and day, those two. Cerra is friendly, easy to handle (grooming, feet without complaint, lead, etc) but she's still pushy and tests boundaries, like any (1horse = 3human) average preschooler would. At this point, she needs boundaries and guidance, with a firm but gentle hand.
The two year old (six human years) is much more interested in learning. She's all "hey! hey! hey! What's that?! Where yah goooooin'??" and is like a sponge, all absorbent, curious, and WILLING.
Also, Lunging as a yearling can have devastating effects far on down the road, because it will stimulate their muscles to grow, when they really aren't finished growing yet. (Horses reach all-round full maturity at seven.) So it would be a good looking horse, until he grew and ripped his muscles. (But you've got a trainer you mentioned, so you're pretty prepared.) :)
There's been alot of controversy over anthromorphizing a horse. (Giving it human emotion). At the same point in time, there are thousands of people in the world who are like: "i love my horse" and really don't know a God**** thing about the animal, inside or out. Horses think and process things too; and if we should learn horse behaviour, why stop there? Why not horse psychology? Learn to THINK like them, and get a better understanding of the world. You'd think that if people understood the way horses learn and respond to stimuli that they'd be respected as more than a chunk of meat. Alas, I am wrong. My BM even told me that giving my yearling a year to be a baby (I'm her 3rd owner; so think about that: birth, mom, taken from mom, new home, new home, new home -- how would that feel for you? She's barely ONE.) is a dumb idea and that I'm anthromorphizing her.
Think about it. This one man studied horses from his own pocket for 15 years. Domestic horses don't understand how to be horses anymore. And in a herd, the Lead Mare doesn't get her place by dominance. She gets it via respect. In the wild, your colt would still be there in the herd. It wouldn't be until he was 2 or 3 to leave. Even then, he'd leave to be with other males. Link: http://www.equine-behavior.com/Introduction1.htm
BE WARNED OF LEARNED HELPLESSNESS!! It's a condition in which the horse becomes reliant on humans, basically. People constantly degrade horses from beautiful, triumphant creatures into, as the BM said tonight: "only a horse. You pack it up for six hours in a trailer to let it out, tack it up and ride it." <-- Is that really a fair life? No wonder we have problem horses. No one gives a **** to learn from them.
Not trying to be offensive. I'm SO steamed up about this.. Also, "Natural" Horsemanship isn't actually natural. We weren't created to dominate them. It was just a clever idea that benefited both parties.. thus, shouldn't it be "gentle" horsemanship? ((My apologies if you read this as attacky. As said, fired up.))