You should only have to do one body clip, and I would do it fairly soon, so it doesn't interfere with his fall/winter growth.
A cheap pair of clippers, like those you can find at WalMart, will not do a full body clip on him. You will need atleast a clipper like an Andis Super Two Speed (which is a dog clipper, although they sell them with wider blades for horses, as well)...these can run anywhere from 120-180 depending on where you get them, but they will do the job much better than a cheapo 40$ pair...you'd probably burn that clipper out after a few swipes.
Another option would be to invest in horse clipper, but those will run atleast 230-290, and upwards of that, however they will get the job done quicker and most efficiently; they do tend to be louder, though, and bulkier to handle, which is why I go for the smaller regular heavy duty dog clippers.
I did a body clip on my mare this winter, and I think it took me about 45 minutes; and she's a nearly 16 hh mare, with some bulk to her! I used one of my dog clippers and a 7 blade against the grain of the coat.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."