Yeah, I get it, my family was sort of like that, too. Not real big on sharing "feelings"--I think it's the Nordic culture, kinda stoic, nobody yells, cries, or hugs.
I think I'd make it short and sweet. Here's the deal, here's what's happening. I think it's important to convey that alcoholism isn't something anyone chooses, that nobody knows why some people are susceptible and other people aren't. You might mention that there's a possible genetic component, so it's important for him to pay attention to his own reactions to alcohol when he is old enough to drink. You can tell him that nobody expects him to do anything special, but that he is welcome to ask questions or to talk with other kids whose parents or brothers or sisters have had problems with addiction, and you can help him find a group if he ever feels like he wants to talk about any of it. You might also give him a heads-up that quitting drinking is very hard for an alcoholic, so not to be surprised if mom seems a little weird for a while.
I think my kids (who are now almost 27 and 29) still think AA is a little bit "weird," but my older son happened to be visiting when I got my 5-year coin, and he went to the meeting with me. It meant a lot to me. Their dad has been sober since before we got married. My older son has a friend who is a chronic relapser, and my ex has been working with him, and my son is grateful for his help. So to them, it's no big deal, but having seen how destructive drugs and alcohol have been for some of their friends, I think they've come to appreciate the value of recovery.