My best tip would be to be happy that he’s developing as he should be. This is completely normal (am just going through it for a 2nd time). How old is he?
I do know what you mean, to feel like they are slipping away. Depending on his age, your role changes here as you go from being our DCS everything to more of an ‘as and when needed’ sort of thing.
You have to work a bit harder to engage them unlike when they were younger and just wanted to be with us. I find myself asking and chatting about things that I would naturally have very little interest in as there can be some lovely moments of connection that would not have otherwise come about. Suggesting things that you know will get their buy in.
If we have 3 or 4 meaningful interactions a day (normally quick chats but 2 way conversation rather than anything deep) then I consider that a good day.
The fact that this is happening means you have done a good job.
You may not feel like this right now but I have found that after years of doing everything for the DCs - whilst them needing us less is a bit of an adjustment- it’s freed up time for me and I feel like I am living more for me and getting some of my identity back. I have joined a gym, choir and now go to yoga once a week and I enjoy all 3 things immensely.
I don’t know your situation re other DC, age of your DS but try and reframe this as seeing it as a job well done on your part as he’s following the natural course of events.
Is he out and about doing things , seeing friends, doing sport or is he mainly in his room? As long as he’s happy and not isolating or depressed / getting up to things he shouldn’t be, then any of these things are as they should be.