Hmmm...well, to be honest, $ for $, a desktop is going to smoke a laptop in points per day, mainly because of the GPU (graphics card) capability. I'm not doubting your technological prowess but please consider running specs of any rig you're looking at by us - that way you're sure to maximize your $$$.
As an aside - you could always set BOINC to run on your son's computer only "when not in use" and he would never even see a difference in performance (disable the screensaver, of course!)
Best of luck, it's a hard decision, I understand that. There's no way we could talk your son through building one from Newegg.com parts, is there? How old is he? Mechanically inclined at all? You would save a bundle compared to an Alienware type rig and we could hook you up with loads of point generating power!
Just something to consider; I wouldn't mind sitting on the phone for a few hours with you or him, walking him through it - it's simple enough once you've done it!