Verified answer

While technically not 'impossible' to add a whammy bar later at great expense, if you want one, you buy a guitar that comes with one.  If your grandson wants a good beginner guitar with a whammy bar, consider a Squier Stratocaster. https://www.musiciansfriend.com/guitars/squier-aff...  

Also consider that if he doesn't already have one, he'll need a practice amp and a cable.  It might make sense to buy a "package" if cost is a factor.  Otherwise, there are better practice amps available, like the Fender Mustang.  https://www.musiciansfriend.com/guitars/squier-aff...

Many/most (but not all) guitars come with one.  It's part of the guitar when you buy it.