Normally they are pretty casual. Shorts and a polo, shorts and a t-shirt, etc. If it doesn't say in the invitation then I'd just dress for the weather (usually hot)
Who finds it "interesting" how many posts USMMA2014 makes asking about clothing and accessories?
I am sure this young person will likely excel in the near future since after he reports for Indoctrination in the next few minutes, he'll have so much more time on his hands since all the time he spends wondering about this will be available for other things and thoughts since it will be easily boiled down to three options: a) Uniform of the Day, b) PT Gear, or c) Boiler Suit.....
Oh and KP2001 in answer to his question isn't a more accurate answer "it depends" on the image you are going for. I mean if he wants to be "emo plebe" would you really suggest the "preppy look" you recommended? After all if your going for some sort of "branding thing" it's important to lead initially with the right visuals. Remember what a total marketing bust "Coke Classic" was....