OP, stepmum here. 5 years in, 3 years married. My circumstances are little different to yours in that I came into my SD's life while she was still only little - you came into your SS's when he was a teen - that's a big difference. A whole different ballgame altogether, and much harder to navigate.
Take out the SS issue for a moment. Do you honestly want to be with a man who just ups and leaves when things get tough? All relationships go through tough times, you don't just bail! How do you know he wouldn't do this again? Can you really live like that? Too scared to speak up in case he leaves you?
I often find myself wondering when parents became such wussy sooks. A teenager should not need reminding to brush his teeth. A friend of mine is a kindergarten teacher and is often shocked at the number of children who aren't toilet trained, or can't wipe their own butts! Four year olds. One parent said to her on the first day of kinder this year "My son has never heard the word no". My friend replied "He will this year!".
When our generation were kids our parents didn't ask us, they told us to do things and we did them. My parents weren't overly strict or authoritarian but they were the parents, they made the rules and we the kids did as we were told. When did it change to kids running the house?
Anyway, that ^^ is neither here or there, it's not the kids fault that neither of his parents have bothered to teach him life skills. The problem isn't the child, it's his father.
I have put a lot of stress on him to fix the issue with his son because I see it as a major issue.
Yes, maturity level wise he is not good for me at all. He is not mature at age 40. However, he does love me to pieces and I do love him just as much back. It is just unfortunate that there is a third person in this relationship (his son) that is affecting it.
That's because it IS major issue. You were right to bring it up.
He loves you so much that he just ups and leaves, skulking away while you're at work because he doesn't have the b@lls to face you?
If I were you I would send him ONE text. Tell him that you consider his leaving to be abandonment of the relationship (because that's what it is), you'll box up his things and they'll be on the verandah at 3 pm on X day to collect. If he doesn't collect them you'll give them to charity. Or take them to his brothers - his family can deal with it. Real men don't just up and leave honey.
Don't sit around waiting for HIM to decide what's going to happen with YOUR life.