This. Four years from now, you will be on your feet crying and grinning and cheering with sheer joy and pride.
You are going through a bit of the grieving process, as you watch the little boy you raised recede into the land of memory. You can't have that little boy back, you can't freeze him in time and space, you can't stop his progress out of the nest. You can grab onto the present and be in the moment with him - but also at a respectful and healthy distance - and let your own path unfold as you watch his with interest and love.
If your mom and dad are still with you, this could be a great conversation with them, because I am sure the memory of your nest-leaving, when it became final, is still seared into their brains.
It's the way of humans. Be a bit sad for the departure of that little boy you know so well, and delight in the man he is becoming before your eyes. You will still see glimpses of the boy the rest of his life.
When we visited my mother-in-law, even well into her 80's, when she was still making some meals for herself in a continuing care community, I watched her smile quietly to herself as she watched her DS/my DH devour a Taylor pork roll sandwich on white bread, a massive pickle and some odd fresh fruit jello thing, when we visited her in Philly. She told me later it was his favorite Saturday lunch as a boy, and when he brought mids home with him for the weekend from USNA. I offered to my husband to stock the makings, but he just said, no, it only tastes right at Mom's kitchen table, and laughed. My sister-in-law, married to the younger brother, also a career Navy officer, shared that the same applied, only with an Italian roast beef sub.
He will always be your son. Just not the same, but someone new to know and love as an adult.