I'm not sure if this has been said, I didn't read the other responses...
Even if he doesn't feel he has a problem with alcohol itself, it very well could lead him back to using his drugs of choice. Easily. With the lack of judgement, impulse control issues alcohol causes....could make it "ok" for him to use opiates. Also, alcohol triggers the "pleasure center" in our brains, which doesn't discern what it is that is causing it to be triggered. It will open the floodgates to using other drugs.
"Only COMPLETE sobriety can keep us normal"
Hope this helps....PM if you need to talk more.