Im 13, and its an age where gifts are going to get more expensive because most 13 year olds arent any longer going to ask for toys, theyll want things like laptops and other electronics. Hope this helps!
How about a good pocket knife. I know I am dating myself but if the parents do not mind I would think it is about the age when a boy can begin carrying a pocket knife or a Leatherman. Again make sure you check with the parents.
I'd suggest an iTunes or iBook gift card, to complement his iPod. Or a regular Apple store card, so he can choose whatever accessories he wants. Speakers for the iPod, a portable boom box, or clock-radio where he could plug and charge his iPod while in use, are a good idea, too.