@MMMMMBacon
Deciding to stay but live separately is a valid decision. A hard one, but still valid.
The thing is, if that is your decision you really need to change the financial set up. At some point DS will leave home and then you are going to want to leave. As it stands, chances are you'd walk away with very little when you walk out the door. Yes, assets will be wrangled over in court and you may very well walk away with your share of them, but how long will that take? And you will have had zero ability to save for your own retirement in the interim.
But in the meantime, living separately, under the present arrangement you are giving him plenty of time to start moving his money around to hide it or give it to family members 'as gifts' with the secret agreement that they are just holding it for him. You'll have a hard time 'reclaiming' money he's 'given away' in the next few years before you separate.
Since moving into the guest room is something he can't ignore the significance of, I'd suggest you tell him that from now on he must pay half the household costs including food and half DS's costs and that he will no longer have access to your money. It will be up to you to figure how you want to work this; either he gives you his 'share' or vice versa if you trust him to actually pay the bills, or open a new joint account and each deposit your share and the bills get paid out of that. I'd suggest either he gives you his share or the joint account. If you can, you want to keep control to see the bills actually get paid.
You can suggest dividing up 'our retirement account' but I wouldn't hold my breath and it may be better to just get some kind of proof of the current balance, hopefully before you lower the boom of completely separate finances.
You also need to give some consideration to division of household labour. If he's now in a separate room then he needs to be responsible for its cleaning and upkeep. And at the very least he should do his own laundry. As far as dividing up the rest, you need to decide what it's worth to you in terms of hassling him to do his share vs just biting the bullet and continue to clean the 'common areas'.
As far as DS, he's of an age to know what he wants and how to 'take care' of himself when he's out and about. If you have 'babied' him, now is the time to start giving him (and you) a little space, responsibility, and a bit of independence. It will stand him in good stead in the future. My sons are in their 30s and the independence we gave them and the trust we put in them when they were your son's age paid off. They're responsible and caring adults with good careers and sound financial 'sense'. DS1 travels a lot for work and he still lets us know his travel plans, mostly because I'm a worry-wart.
I disagree with putting any kind of AirTag in his bag unless he agrees to it. Much better to have a quiet chat with him about the importance of him being contactable and that letting someone know if one is going to be late is a common courtesy for one 'adult' to give to another adult. FWIW the basic rule we had for our sons was that they tell us where they were going and when they expected to be home. They had mobiles but this was before 'smart phones', so we asked that they let us know if they went somewhere else or if they expected to be late. Again, we phrased this as safety and that it was a courtesy from one 'adult' to another.
It's a lot to think about. But also this: even if you decide to stay in the same house, it will not hurt you to see a solicitor to discuss what protections or vulnerabilities you may have as a 'separated/living together non-couple'. And what a divorce may mean to you should you choose to leave.