Wow - people on this board can be really merciless and incredibly condemning and instantly start pushing people to divorce based on a paragraph of information - after 19 years of good marriage to someone op says has been supportive in every other way..... and all for what? A problem that has emerged in the last few months, and is expressed no more seriously than with a bit of sulking?
I find this reaction unbelievable.
Marriage is surely meant to be an emotional, committed relationship in which when difficulties come up, you seek to understand, talk and work through things, and support each other? That does not mean the behavior should be accepted and of course the son is the ultimate priority. And absolutely, this behavior needs to be resolved. But that is different than chucking in the towel at the first sign of a problem!
The husband is obviously having difficult feelings, and is expressing this in a pretty juvenile way- but the feelings themselves are worth caring about. 'Rivalry' with the son is only the symptom.
OP- you are in a long term marriage - your husband may well be struggling with emotions about growing old, and perhaps your romantic life is a bit stale- perhaps after raising six step children with you, he has been looking forward to time with you that is exclusive - perhaps he feels he doesn't have enough of that, and still - even when your son is grown and ready to fly the nest, you are overly focused on him/ not letting him grow up/ move on.... perhaps he feels (and is maybe even treated) as invisible? I have no idea if any of these things are the case ..... but my point is that a man who has been a positive supportive husband for two decades deserves more than divorce papers after a couple of months of the sulks..... I hope you don't feel pressured by the posters here to take action you would never otherwise have felt was right for you.