What level show is your son competing at? Usually county-wide shows or smaller, western G&S guys wear pressed dress shirts (western print or plaid is pretty common) and nicer jeans, also pressed, plus cowboy/riding boots, cowboy hat, belt, and maybe a bolo tie. Regional show level or higher, sub dress pants for jeans and add a blazer. Here's a good chart for reference for about all classes: Hobby Horse Clothing Co. - What to Wear?
As far as length of jacket, I'm not too familiar with how guy's blazers, etc. should fit, but my western show coats fall about a hand's length lower than my hip. Honestly, I've never seen a boy in a G&S class in a blazer in 7+ years of showing 4-H.
Guys have a little more leeway color wise to switch things around. Gals want a solid base color silhouette with pattern accent, etc., but guys can get away with blue jeans and a red shirt (lucky ducks
). A guy friend of mine shows in blue jeans, reddish-brown leather cowboy boots, and a white western shirt with a blue and burgundy yoke pattern across the shoulders. He has a chestnut horse.
Here's the color wheel as well, to get you started on complementary colors for such-and-such color horse: Hobby Horse Clothing Co. - Description
The Hobby Horse website in general has some good tips for the nitty-gritty details that can be helpful. Their clothes are rather pricy, and I don't think the men's department is that extensive.
Good luck to your son, and remind him to smile and have fun!