Well... that stinks. Any which way you view it.
So... let me ask you this: When he quit drinking -- did he have the help of any program? Did you have the help of any program?
It seems to me a lot of recovering alcoholics for some reason get to that point where they think they're "over it" and can handle moderate drinking. And then *poof* they're right back in that hell that is active alcoholism.
I guess for me, I think before saying anything to him, I would want to sort things through in my own head. What does it mean to you if he continues actively drinking, as an alcoholic? What can you expect living with an active alcoholic? Is that a life you want for yourself, for your son? If not, what are your options?