Revan can drink fine and doesn't get drunk, Ever, Xavier is too young and probably couldn't handle it anyway, Julian stays away from anything that could alter his state of mind, so he doesn't drink, and Aiden is just unpredictable. Sometimes alcohol has no affect on his actions, others it makes him go on a rampage, drown in self pity, go into a comatose state, and who knows what else? Basically, Aiden just shouldn't drink, which means it's almost inevitable that he will. Especially with the people he has as his partners.