Editor#5
It is not uncommon for two students to go together as dates to Midwinter strictly as friends. You seem to see Midwinter as an opportunity to go with someone who is relationship material — an opportunity that is being eclipsed by your guy friend. This smells like a predicament brewing with awkwardness. You two have probably worked out some sort of deal out since you posed this question to us.
There are a few things I’m unclear about. Do you already have someone else in mind that you were planning on asking? Did this guy ask you in a thoughtful way or hastily? Do you even want to go to Midwinter at all? These are things to take into account. If he actually took the time to plan his asking versus just shooting you a single “Midwinter?” via text, then it is harder to reject him. The best solution is to talk to him face-to-face or even on the phone. Texting him about the situation is the worst option since you cannot hear his voice or see his body language, which are key factors in determining whether he will be hurt or not by your response. Depending on how his body or his face reacts to your words, you can adjust your tone or what you need to say in order to convince him that you are not the one for him.
Perhaps the reason he asked you to Midwinter is because he did not understand the first time around that you are not interested in him. Your “making it clear” about your friendship may not be the same as his “making it clear.” I do not want to read into this too much, but perhaps this guy lacks some social intuition and only starts listening to people the fifth time they say something. If you have to give your friend some tough love, do it. Tell him as many times as it takes so that he knows for sure that you do not want to be romantically involved with him.
Technically, Midwinter is a Sadie Hawkins dance, in which the girl is the one who asks for a date instead of the guy. If this guy is persistent and still wants you to go with him, your last resort may be to tell a white lie. You could tell him that you already planned out a way to ask somebody else and that that going with him will cost time and money you do not want to be wasted.