I know exactly what you mean. The final parting I made from my parents home after graduating from college to move far away was very difficult. My Mom was sobbing openly and my Dad and I were teary eyed but trying to remain brave. Several days later, my parents wrote me and told me it was one of the saddest days of their lives.
Many years later, near the end of my Dads life, I had the pleasure of making audio recordings of his narration of important events in his life and we revisited that day I left home. I was surprised and elated to discover he remembered the day not with sadness, but with great joy and pride. My life had been filled with blessings and excitement that I shared with him throughout. He remembered the day of our parting only as the joyful beginning of my adventure.
I hope that God grants a similar happy remembrance of this difficult time of parting to you and your son.
Thanks for sharing your final parting story. God Bless you!