It can depend really, on a lot of things, how much he's drinking, if he's eating too, how he reacts to it, carb content of the drink etc etc....
The main thing to remember with alcohol is that the carb content will have an initial effect on blood sugars, but the alcohol content of the drink will most likely lower the blood sugar later. This is because breaking down alcohol 'keeps the liver busy' so that it's not spilling out it's usual basal rate of glucose, the amount it does this depends on how much is drunk, so sometimes you may find that you don't need to inject at all for insulin and need to snack after drinking.
If it is just one or two quick drinks before dinner I would most likely not inject for it and just correct with my meal, others may have a different approach though. As with so much it's trial and error really.