To continue with Gerrit's idea, having your son sit by the radio recording stations he can hear might be difficult if his time on the radio is limited or if he can't do repeated sessions under similar conditions.
One thing that might help is to collect data on contacts that are posted on the internet and used that to support or refute his hypothesis.
One website that might help is http://pskreporter.info
. There you'll find digital mode contacts around the world broken down by band and time block. The data is plotted on google maps that might make a nice display if your son is making a poster.