software download blocker

Simply; My young fella likes to download junk to my computer and its
not worth getting into a fight over it so I would like a hidden program
that will allow him to browse (for educational purposes etc) but not be
able to download any files.If he needs something that bad, he will get
it from a friend which is fine, just not on my computer, sometimes he
doesn't think, we are working on it :) Am presently running AVG anti
virus with firewall but doesn't seem to have that option. Any
suggestions please. Many thanks...