The facts:
He is/has homosexual(tendencies).
He has a drinking problem (and there is no proof, at least in your posts, that he drinks around the minor child.)
The assumption: That he might be exposing your minor child to the lifestyle that he lives. (I don't see any proof of this, nor is it something you should be concerned with, since as far as you know, it isn't happening..
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If you don't really think your son is in danger, he's probably not, and you're better off not asking a bunch of people on the internet (how do you know who we are, any better than the people your husband talks to?
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It IS unfortunate that your daughter found out that way, but she has nobody to thank but herself, because she snooped. At 20, she's a legal adult, and there is no excuse for that. AT all. That said.. If your ex didn't tell you, it's really none of your business what his sexual orientation is, nor his activities, unless you have PROOF that he is causing detriment to your minor child with them (which to this point, you haven't posted any).
It seems to me that you(and unfortunately, some of the seemingly prejudiced posters in this thread) are more concerned about his activities because of their "same sex" nature, rather than because of his promiscuity. Had he been sleeping with women, would you be nearly as concerned? I'll answer that one for you.. Yes, but not nearly as much.
A few posters also mentioned the fact that your minor child probably shouldn't find out about this right away.. I agree, but that's something else I didn't see you post that he intended on doing, and that's another thing I think you should leave alone. If he's competent as a parent enough to keep him 3 days a week, he's competent enough to decide when the right time to tell his son - whether you agree with that time or not.
Feel free to discuss your feelings with your ex... WHEN you find out in a non-intrusive manner (ie. him telling you, not someone snooping on him), or WHEN you determine that there is actual danger with your minor child involved.. Until then, your best course of action is to mind your own business.
Oh.. and if you're not a churchgoer, use careful discretion in following the advice posted above to find one.. many churches are 100% intolerant of homosexuality, and you might find yourself being urged to remove your son completely from his father's life, purely to satisfy a religious agenda, without any true consideration to your son or his father.. I can't say ALL churches are like that, but a good amount will be.