My mom and my dad both remarried and I'm left out of the wills for the most part due to being the oldest and already being on my own. I'm 25 with a car and some debt and a career that I am not finding enjoyment in anymore. First divorce happened when I was 4 and again when I was 18. I have 2 sets of step-siblings of varying ages who are included in the wills of their parents.
When I was 17 I learned I was left out of pretty much everything except the old ass Gerber life insurance policy on me. I felt hugely forgotten and not cared about. I already had to deal with trying to divide my time and life in two to see my different families but now I felt like neither family truly accepted me as one. Slowly the phone calls from my dad and mom would grow farther apart or they wouldn't answer or would cancel plans with me. I didn't know why I was never invited to birthdays, I didn't know why I was left out of my step-sisters wedding. I didn't know why my first step dad was the only one to call me on my birthday.
Learning of my placement on the will was only the trigger that brought out those emotions. I had long felt neglected by both sides of the family and watching both, my Dad and Mom start new lives and be happy without each other made me feel like a mistake. I felt incomplete, like a project that was started and never finished. Those feelings were brought out by reading the will. Even though I thought I had dealt with those emotions in middle/high school they bubbled to the surface again and brought me great turmoil.
I did almost the same thing your son is doing although I didn't say anything to my family beforehand. I left, I found a friend to stay with for a couple weeks and shortly later I moved out and have been on my own since then. I distanced myself from both sets of families and before I knew it I found myself truly alone and in a dark place. I was finding my love for recreational drugs and alcohol abuse. I would routinely kill 24 packs of keystone with my roommate in a night. I was dating women who were completely wrong for me, and was becoming more and more depressed.
I was so torn between being happy for my parents and their new families and being resentful for not being included. My mom loved me the same. My dad loved me the same. I loved them as well but there was an internal conflict eating me up. They saw me as an adult and doing well in my career. I was still living like an angry teen oblivious to what damage I was doing myself to the relationships I had with them.
My parents didn't want to hurt me by leaving me out and I knew that. However the mistakes I made afterwards are what solidified my role as the black sheep step-brother who doesn't ever have his life figured out. I should have talked to my parents then instead of running. I harbored feelings of neglect and rejection for no real reasons in the end and have spent the latter part of 2 years trying to repair the damage done. I may not be in the will, but damn if I won't be part of my younger brothers lives going forward or be there when my step sister has her baby.
I understand this is more of a personal re-telling but I hope my insight helps you talk to your son and understand his pain and work through this with him. He knows you love him, however he is probably still reeling in feelings of rejection and loneliness. I believe these feelings have likely been around for awhile and he's embarrassed, confused and feels like his lifes inside out right now. I think we all know that adulthood has worse experiences around the corner and from my experience I suggest you tackle this problem early.