Does he prefer to work with a mouse or would something that is more text-centric be OK for him?
If he would prefer to work with a mouse, I would suggest that you get Math Type for his Windows or Mac systems. That even lets him input mathematical formulas using the mouse directly (see web page).
If he would rather enter text and have it render as mathematical formulas, lyx might be suitable for him. It is an editor for LaTeX that has really nice features for entering formulas. It is available for Ubuntu.
I would personally go with the second option, but I am biased due to working a lot with LaTeX.
As for drawing, I would recommend inkscape -- it's a drawing program that you obviously operate mostly with the mouse, but everything (line lengths, positions, colors, etc.) can be edited with the keyboard as well. It also has really good support for aligning objects in the drawings, so mouse precision is not needed either. This may be particularly good for him if keyboard-centric applications would suit him better (related to your comment).