To make life easy never wash all the sheets at same time. So kid’s sheets one day king sheets another day. Then you can keep sorted sheets sorted.

For the future buy different colored sheets for each bed.

For now label every tag with different sharpie. So black for king, red for twin etc. Or if you can sew sew a tiny ribbon tag or dot several places on the sheet to make it obvious what size it is.

To store put in clear spaces so king on top shelf and twins on bottom etc. or is separate bins or whatever makes sense so it once sorted you know what it is.

To make sets of sheets make them into sets so put a set in each pillowcase or fold set into and wrapped by a sheets.

Add very light dusting of powder or baking soda or flour on things he could touch that you want to know. If he touches it you’ll be able to tell. Make it super light dusting so it’s not obvious at quick glance.

Put up paper sign With ton of tape blocking use of it marked private so it can’t be opened. Any guest should be fine with not touching anything private.

Put something super small in very distinct way you’d remember so if it’s touched you’d know.

So if you put baking soda all over floor and sign it with your finger so he can’t replace it in room he’s not supposed to be in he can’t ever complain if he sees that as he’s not supposed to be there!

I’d also do indoor motion sensors to make sure they are not in the house or can access house with existing keys too.