When I was 10 I was learning BASIC on an ATARI 65XE by transposing code from the manual into the computer. Then taking that program an moding it to do more / other things. It was fun, but then faded off until my family got a 286 then I was working in batch. It is simply amazing how much you could do then with autoexec.bat, config.sys, menus, and some .bat scripting. I still use some of that knowledge today.
Then I moved onto cheating in school with my TI-82 by writing and disttributing code to my classmates that would prompt for info and give the results. Was great for the math classes that required the showing of work, and could instantly see if my answer was correct or not.
Also came in handy in Biology and the periodic table. Was more of a db then a program, but I didn't understand dbs then.
Now I'm comfortable working in C, C++, VC, VB, ASP, PHP, SQL, and a few more. However, until I have reason to belive otherwise, I will not use Java or any of the .NETs. But that is for a different conversation.
Now, back to your question. whatever you feel comfortable learning too would be my answer. He will have questions that will need answered. Be ready with answers or be ready to do much research with him to develop the best solution as possible for him. There is nothing worse then trying to do something and finding a brick wall that without real-life interaction will not be able to be fixed.
Thankfully today there is this wonderfull world we call the internet to assist. I wish I had this when I was learning (well I'm always still learning....but I digress). All I'm trying to say is that there will be a point that he may / will get frustrated in doing research.
BTW...good luck!