I can understand your point here.  I too was in a similiar situation with a bipolar who drinks but he is a safe drinker.  He takes meds at bedtime but also he drinks beer but mixes n/a beer with his regular beer.  He no longer drinks the hard stuff.  It can be  a problem here.  I used to drink myself and found that medication does not work effectivelly if you do this.  I quit drinking and smoking.  

Maybe if things get so bad you could have him committed on your own.   Sounds to me he is just having fun with his friends or wherever he is drinking.  Maybe try and get him to drink at home so you can sort of counsil him more.  If he is an alcholic then you best get him some help cause the bipolar will eventually get him in trouble.  I know you love him but don't forget to love yourself too.  It is difficult esp. with children.  He won't go to AA? Does he ever want to quit?  Maybe you could join Alanon and get some better tips.