It could also be something like low-grade depression. Lots of alcoholics are actually self-medicating (of course, alcohol is a depressant itself, so that doesn't work great). But the alcohol can mask the symptoms.
If he's willing to see a psychiatrist or a psychologist for an evaluation that might be a way to start sorting it out. He's a lot more likely to get an accurate evaluation/diagnosis without the alcohol messing with his thinking.