If both of your names are on the Lease or if his name appeas on a deed, and he gave them permission to search, they were legal in searching the entire property, not just what you consider "yours" when talking about a roomie.
Is he over the age of 21? If so than they cant get you for underage possession, unless he is denying that the alcohol is his. If this is teh case, your roomie is out to screw you hardcore.
If he is under the age of 21, than he should be charged the same as yo with possesion by a minor.
I am no lawyer, but that is my interpretation.