What you're describing is completely normal.
First of all, don't ask "how was school" - you will only get "fine" or "I forget." Learn to ask more open-ended questions. When he gets out his homework, ask him to show you how to do it. That will tell you what he learned in school much better than "How was your day?" or "What did you do today?"
Secondly, he's moving into a new and important phase, which doesn't mean he doesn't need you. It means he needs you for different things. You're going to miss all of this if you don't see him as just as important at every age as he was as a baby.
A parent should not be the child's entire universe. If you look through other posts here, you will see parents upset that their children are crying at school, or unable to be in a room alone, or unable to sleep alone. Is that what you want? Of course not. But you do seem to be upset that he has confidence, independence, maturity, social skills, friends, and the ability to separate from you and connect with a teacher, principal, and other adults.
You don't have to emotionally let him go. You have to let go of the toddler and the preschooler, and move into the role of "mom of an elementary schooler." If you don't master this, you will have a terrible time with the tween, teen and college years! He hasn't been a baby for a long time, but I think maybe you missed that part? What's going to happen now is that you will miss all the wonderful moments of child & mother because you're only focusing on what you don't have, rather than what you do have.
Honestly, I think you've done a great job if he's able to navigate these new experiences so well. Please read some good books - ask your local librarian - on different developmental stages of kids' lives (recognizing they come at different points with different kids). And instead of seeing him as "starting to pull away", see him as "joining the world around him" and "needing more guidance from his mother in different ways." I know you see this as "wrong" at age 6, but at what age would it be okay? At what age would you suddenly be willing to let go? 8? 10? College? The point is, it's never easy if you wait for a specific cut-off point. This is a continuum. He's on it. You can join him on it instead of lamenting that he went ahead while you weren't looking.
We all miss things about our babies, then our toddlers, then our kindergarteners, etc.. It's okay to see milestones as a little bittersweet - but it's not okay to fall into abject misery because we underestimate our roles. We all need to learn to love the entirety of our children.