If he hasn't started to shed AT ALL, you might get away with it. But if he's even losing just a few hairs, the summer hair tips are through the skin. You risk clipping a small portion of the summer coat if shedding has started.
NO idea what the official rules of clipping are, but I had a clinic in early April last year and I body clipped my shaggy, horribly shedding disgusting mess of a horse around March 1, so the clip job would have a bit of time to grow out before the clinic.
Didn't seem to impact his summer coat - he was just as dapple-y, shiny and gorgeous come end of May/June as he normally is.
You can clip them in the fall, you can clip them not at all, you can clip them dead of winter, their summer coat you will not hinder, you can clip them any time, their coat will be completely fine! You can clip them any day, their summer coat won't go away. A little curry, Sam I Am, and they will shine for sure, yes ma'am.
I'm with deltawave. I grew up riding at a big name show barn that did WEF and the like. Hunters, eq horses, and ponies were clipped (if they needed it) if they were showing at the big A's, period. It didn't matter what time of year it was, horses and ponies got clipped all the time so they would look the part, and they always did.
My horses are getting another clip job in the next couple of weeks, too. I wouldn't worry about it. I've never seen a February clip job ruin a summer coat... Worst case, you can always clip again come spring.