1. A 'real' computer - or just let him use the family PC - install Minecraft. Play with him and encourage him to build things with redstone.
2. Lego Mindstorms - build robots with him, discuss cool things they could (reasonably) do, sit down with him and write the code.
3. Get familiar with the basic logic gates and make a few puzzles. Build an adder, hook it to some switches and LCDs. Make a project out of it.
4. A good algebra book.
5. A TI-83/86/whatever. I wouldn't go with the NSpire, though. He'll be able to make programs that scroll "BUTTS" - he'll love it.
That's my advice.