Why isn't your son satisfied with the old Dell laptop?  How old is this laptop?  What does he want in a new laptop?

Has he tried using antiX Linux or Puppy Linux on the old laptop?  You haven't told us anything about the hardware specs.

Although several other people have suggested a used laptop, I'm not sure that's such a great value.  Laptops depreciate much more slowly than desktops but are subject to more wear and tear and have a battery that costs a lot of money to replace.  Compare the prices of the used laptops with the prices of new laptops.  At least you know that a new laptop has no prior wear and tear and has a brand new battery.  I was considering buying a used laptop 4 years ago but ended up buying a new one because the used ones weren't really cheaper unless they were very old.

I like the idea of running Windows in Virtualbox better than dual booting.  (I haven't gotten around to trying this myself, though.)  If the Windows installation gets messed up, your son can just create a new virtual machine without having to tie up the whole computer with a Windows reinstallation.

-- 
Jason Hsu, embedded engineer, Linux user <jhsu802701 at jasonhsu.com>