Alcohol is not unlike any mood altering substance in that tolerance is built and it takes more and more to achieve the same "buzz", so he hasn't gone from drinking a lot to suddenly being almost fall down drunk from several lite beers.
Well, as an alcoholic myself I can tell you that this isn't necessarily true - though it may be. No way to know.
There is a phenomenon called Reverse Tolerance in long-term alcoholics and it is when there has already been damage done to the liver/organs. You can Google that.