Welcome, I'm sure you'll get a lot of help here for all you need.
Is there a particular reason for going with GIMP 2.7.x (the unstable version) for your son's workstation? Depending on your/his familiarity with the program I would suggest starting with the normal stable version. Once things are set up for that, then you could expand from that.
I don't know what kind of art your son wants to do, but at least in the case of digital painting, (which is what I do), I would recommend a setup using MyPaint and GIMP in the first run. Both are open-source, free programs although with different focii. You found GIMP already, MyPaint is over at
http://mypaint.intilinux.com/.
If your son is, as you say,"100% disabled" I imagine he would need some sort of special input methods in order to use his workstation? What operating system are you using, which platform? I unfortunately have little knowledge of what input hardware is used and how these programs support them.
.
Griatch