Hi, blob,
There's a lot of pretty cool stuff (Escher, tesselations, fractals, paper-folding, puzzles, etc.) at
http://mathartfun.com/shopsite_sc/store/html/index.html .
Kriston's right, pattern blocks are very fun! Also pentominoes, tangrams, even geoboards.
Is your little guy crafty at all? There's just nothing like weaving for setting up interesting patterns. We have one of those little funny potholder looms, which has been fun, and I also have a floor loom, and some other little ones (inkle, tapestry, etc.) Card weaving is also fun, and very cheap in terms of equipment. There are a couple of pretty good weaving books for kids out there--I'll get back here with titles ASAP!
A math book my pattern-loving Harpo has really enjoyed over time (it's one you can keep coming back to) is Don Cohen's little calculus for kids book:
http://www.mathman.biz/ It's totally about discovering number patterns (it's not very colourful, and the website is a bit zany-looking, but the books themselves are really meaty and interesting).
Hope that helps--
peace
minnie
ETA weaving resources: This first one is my favourite one: Kathleen Monaghan's You Can Weave: Projects for Young Weavers.
http://www.amazon.com/You-Can-Weave-Projects-Weavers/dp/0871924935This one has a bit trickier projects: Kids Weaving: Projects for Kids of All Ages, by Sarah Swett.
http://www.amazon.ca/Kids-Weaving-Projects-All-Ages/dp/1584794674I really like Elizabeth Jensen's Small Looms in Action, but it has been out of print for some time, unfortunately; your library may possibly have a copy.
Children's looms:
http://www.bountifulspinweave.com/kids_page.htm