Yup, mine is 15 now and the year before last for school shopping he decided that he wanted all of things from a couple of expensive store that we normally didn't shop at.
I explained the budget to him, outlined what he needed, then we checked out prices. Dd he want 2 expensive shirts, or 4 medium priced shirts? In other words, did he want to wear the same 2 shirts all week, or have a large enough wardrobe to cover the entire week. He decided on the larger wardrobe.
Underwear and socks I buy when he is not with me - I just buy new from walmart or target and put in his dresser drawers. He probably thinks the "Underwear" fairy takes care of it.
Last year I was unemployed and major back to school shopping was not an option. I spent a lot of time in thrift stores picking up clothing for him. I never took him with me, I would just hang the clothing in his closet - he would find it, and I would just tell him I got him a new shirt. etc. He adopted a "don't ask, don't tell" policy last summer. He didn't want thrift store clothes, but he knew he needed clothing.
Your son is experiencing angst about fitting in at college. Normal, but frustrating. I would be more concerned about his being rude to you than what kind of underwear he wants. Buy him the Hanes, and let him know that he can buy his own designer undies with his spending money - you know, let him pick between food or Calvin Kliens once he is away at college. I bet he picks food!!! If he really doesn't like the undies - he can always go "commando".
Do what the other posters suggested - give him the money, let him go shopping. When he gets to college and realizes that he only has three days worth of clothing, he will learn a valuable lesson. And learn to do laundry!!!
But, do call him on his attitude. A young man of 18 should never treat his mother like that.
Good Luck
God Bless