Yorkiemom is right--bring black or gray. Something dark, as my son ended up tossing the issued white briefs after Jacks Valley as they just got too muddy, dusty etc to come clean. Same with some of the white socks they get issued. I went up for A Day and took some replacements.
My son wore one pair of compression shorts on I Day and took 2 others. arewethereyet, if your son got to keep 5 pair that is great! I would say not to push it by taking too many, or the cadre may not let you keep them, but a few pair should be fine. Basics also will have an issued bathing suit that some cadets wore under their ABU's if they didn't have compression shorts to help prevent the chafing.