Hello,
I've been on Effexor for a few months (though not as long as your husband) and I haven't experienced an increase in forgetfulness. I don't believe that it is mentioned on the manufacturer's list of side effects, either. However, every drug will effect every person differently.
Could it possibly be that since he's been taking medication, his depression has subsided, and he's become more involved with people and activities in his life? When my depression was at its worst, I didn't want to do *anything.* Now that I feel better I have more on my plate, which greatly increases my chances of forgetting something.
Forgetfulness can also be a symptom of other mental illnesses than Alzheimers, like ADHD. Of course its unlikely he would develop ADHD at 40.

It may be a good idea to take up the question with his doctor. Best of luck!
Espresso