As a fellow junior racer, I'd say your son's best bet for sponsorships would either be to hop on board a
local team or create your own team(!). It's very hard for a junior racer to individually get sponsorships, but if you're on a unique team it's a different story.
A couple of friends and I make up our local junior team, "Team Atrocity." 2007 will be our first year racing under the name Team Atrocity, before we had each been racing unattached or with a low-level club. So far, Team Atrocity is coming together very nicely with the help of
American Classic,
Kenda,
Marzocchi,
GU, and
ODI.
Here's our team resume. It's a draft (excludes some pics) - for some reason I can't locate the final PDF we sent off.
http://www.goatrocity.com/docs/Resume.pdf