"S Mode" (as opposed to Windows S which was briefly a separate SKU) in current builds of Windows 10 (Home/Pro/Enterprise) is now arbitrarily admin activatable/deactivatable. As your father's admin (assumedly in this example) you deactivate S Mode, install all the old apps, then reactivate S Mode. Your father still gets access to the old apps, and can install new apps from the Windows Store [1] without your involvement as admin. You just have switch it out of S Mode if your father finds another old CD or floppy behind the couch to (re-)install.
[1] I don't recall if S Mode currently allows sideloading non-Store but code-signed APPX/MSIX packages. I think it is supposed to? But I think my confusion is that it may differ (at least currently) between Windows Home in S Mode and Windows Enterprise in S Mode.