You could replace the tower for less than messing about with additional memory which might not fix it for long as everything online gets more and more intensive.
Consider a refurbished tower with at least 8GB of RAM. A new screen would be better viewing but could manage without so the parts actually in use will seem the same.
Windows 10 is similar enough to Windows 7 to not be more than a minor issue at first.
Late 70s is not *that* old and the time he has left is too precious to sit waiting for Windows 7 to load stuff  ;)
I know plenty of octogenarians more capable than me online but do concede they don't like changes.