Tom,
It IS true that AMSAT is always looking for volunteers. And we have an amazing number working on projects ranging from Fox to the day to day operations. However messily, the recruiting process works. However, in most cases it comes from someone with a specific skill, contacting a specific individual, about a specific project. In other cases, the person needing help contacts someone who they know who is a good fit, or based on a trusted recommendation. This in many ways mirrors corporate and educational hiring.
What you are talking about are the generic "I want to work for AMSAT, what can I do?" volunteers. As has been said, those reading the applications are also volunteers who pass them on to other volunteers who may or may not respond. There have been times when the press of other projects, or personal health matters, have caused the applications to fall through the cracks. Also, as Gould has remarked, the idea of working for AMSAT is often more appealing than actually working for AMSAT. This is no reflection on your son or anyone else, but it does tend to encourage the use of people with proven skills and record over "walk ins."
As another respondent has said, ITAR is a complete roadblock for non-US citizens for most areas. As a result of the NDAA changes, the first draft regulations are appearing. We have volunteers plowing through the extensive documents, most of which seem to be written in a foreign language. However, we have expectations that the current ban will be lifted in the foreseeable future, or at least made more reasonable. The primary benefit will be working with our colleagues in other national AMSATs, but it will also open up our own ability to accept foreign volunteers.
All that having been said, I am working on getting the AMSAT generic recruitment running a bit more effectively. This will include a more targeted approach for those expressing interest, and at least some sort of reply. Of course, I am also just a volunteer. :)
73s,
Alan WA4SCA