I mean this kindly, OP, but you really need to start letting go. You need to accept the fact that your ds is an adult now, and the truth is, he won't need you as much as he did, because you have effectively raised him to be a confident, independent young man who is capable of standing on his own two feet. And that's wonderful!
The transition into adulthood doesn't mean that the relationship ends there, though - it just evolves gradually into a different sort of relationship in which parents are still needed for support - emotionally and often financially - but the child inevitably starts taking the lead, making their own decisions and acting more independently. And we have to let them do that because that is exactly what they're supposed to be doing at this stage of life. Would you rather see your son lacking in confidence and afraid to branch out on his own? Of course you wouldn't.
I think the key to maintaining good relationships with young adult children is respecting their growing independence and the transition to adulthood while letting them know that you're still there for them if they need it. My dd wanted us to drive her stuff up to uni but didn't want help unpacking etc so we stayed in a hotel overnight and met up with her for lunch the next day. It was fine, and it was what she wanted - and it was her milestone, not ours.
Of course, you're right that this will be a big transition for you too, and it's sensible to plan for this, but I think you're focusing on the wrong things - you're focusing too much on the stuff that is actually your ds's stuff to plan, instead of thinking about how you're going to find meaning and purpose in your own lives once he has gone. My own mother had a major depressive episode when faced with an "empty nest" and being very close to my dd, I was worried that the same thing might happen to me, so I planned as much as I could to try and avoid this. Maybe you could start thinking about what you want your life to look like after September. Is there a new hobby or project you can start working on? Volunteering? Career progression? Can you plan more time in with friends? Anything that you and DH could do together? What is this next phase of your own life going to look like?
I understand that it will feel like a big change. I was incredibly close to my dd and worried that I would miss her terribly. And I do miss her, but actually it's fine. She is home for holidays quite a lot in any case, and we stay in touch by phone/text and visit occasionally. And in any case, you adapt to the new reality. DD and I are every bit as close as we once were, it's just a different relationship now because she is grown up, confident, capable and pursuing her own dreams - which is all I ever really wanted for her anyway.
In your shoes, I would stop stressing about the logistics for now and start planning stuff for yourselves to help ease the transition. It is actually very nice that your ds wants to help his friend move in - what a kind and thoughtful young man you have managed to raise. I would try to facilitate this if you possibly can. Think about it as a way of building the foundations for a new and different relationship with him as an adult that will hopefully continue far into the future.