Thank you Eruraion. That fully answers my question.
However, I just stumbled upon this Switch Between Versions - Howtogeek
From the looks of the tutorial, it seems pretty straight forward. He should be able to keep playing his latest version, which I'm sure he has a lot of stuff built that he wants to save.
And then, create a second profile and signify that it should run the necessary older version. Then, if he wants to play mods for older versions, he just selects that profile.
If done correctly, I assume the stuff he built, etc... in one version will not show up in a different version? Which is completely fine.
Thanks again, I really do appreciate all of the help. You guys are going to make an 8 year old exceedingly happy.
Edit: Will post in appropriate forum next time. Sorry.