TweetThat's tough pally. I feel for you. 20 is definitely to young to decide whether or not you want to settle down for the rest of your life.
Here is my honest no joke answer. When my wife and I were having issues a little while back, we were going to individual counselors in the beginning. My counselor said "Most marriages would be saved if people went to a marriage counselor BEFORE they got married!" I was curious, so I asked What he meant.
Very simply, he said "If I asked you to name 3 reasons why you wanted to marry your wife before you got married, what would you have said?" So I did the usual "Personality, looks, sense of humor...." He just went into detail on how much more there was before I got married that I should have known. Like "After we're married, and we get into an argument, are you the type of person to sit and argue it out, or leave?" There were tons of things that he brought up that I said "Holy shlt!" Had I went to a shrink before we got married, I may not have married my wife in the first place!!
Consider that option with your son and his fiance. Sweeten the deal. Tell them you'll give them $XXXX towards a down payment on a house if they complete pre marriage counseling.