a) whatever you decide in this situation, won't be "wrong" it's not a moral question, and you are not responsible for his, or his parent's feelings. but it will turn out well, or not, for you.
b) I don't know about you, but I rarely have ONE motive for doing anything: just because one of my motives for doing something is less than saintly, does not mean that it is not the best thing for me to do for my own recovery, in a situaiton where I am unclear, sometimes waiting helps. sometimes it makes things worse. but in the end, if you blurt out that he is an alcoholic and you don't know what to do,how other people recieve that may be painful to them, and may not help you, but it won't be a wrong thing to do.
c) I have never had good outcomes from confiding in ex Ah's parents, nor from NOT confiding in them, I have been blamed for telling them, not telling them enough, and hiding things from them. My place is in the wrong, and my actions have no bearing on that, so you may as well do what you can live with.
good luck (())