Maybe a bow-tie block? Really, though, piecing involves a lot of math. I would think anything detailed would be pretty appealing to a math person.
Depending on his sense of humor, maybe you could do a Money Tree. Couldn't find a great pic, but here's a rough idea. Just make the circle shaped leaves silver/gold/copper instead of green. You could even applique/embroider monetary amounts on them.
http://www.crystalpalaceyarns.com/eq...-pfpcolor2.gif Maybe print one or two with his photo & the saying "In XXXXXXX we trust" like his own minted coin. Or if you want to be really geeky, put Gene Dodaro's name/photo (head of US-GAO) or a pic of the CEO of the accounting firm where your grandson hopes to work someday.