Well there could be several reasons why he does this. First of all, (and the rest of you should take note), it could be because he is trying to show respect for those who are paying for him to be in college. Do you know how many freshmen drop out because they partied too hard? This isn't high school anymore. Not that I'm saying that you SHOULDN'T have fun, but remember, once you get out of college you have to take responsibility for yourself.
Secondly, like other people have mentioned, perhaps he is too busy with school work. Thirdly, he could've had bad experiences with alcohol and could be trying to kick the habit. Lastly, maybe he just plain doesn't like alcohol. Rare as it is, it exists (*points to self* :P).
You could ask him yourself, but do not push him if he says no.
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Don't be afraid to stand up for what is right!