Dunk, how fantastic for this dream of yours to be finally realised.
And oooops seems I forgot to put my link in. It's in there now.
Jon, I've got a 10" scope, but I've been doing a lot of observations with my 4" newt. in the last 6 months. It's surprised me no end how much is visible despite light pollution thru this scope! I'm sure your son will have a great time with that 5".