The only thing that you need to be able to access the the new forum is a yahoo account. There are many different ways to get a yahoo account, one of which is to sign up with and download yahoo messenger (which sounds like what your son did).
The direct link is
http://groups.yahoo.com/group/FRCtech2002/
If you already a yahoo account, such as through yahoo mail or messenger, you can click the join this group button and sign in. If you don't have an account, when you click the join this group button, it gives you the option of creating a yahoo account.
There is also a way to join by sending an e-mail to
FRCtech2002-subscribe@yahoogroups.com I have not tried this, so I don't know if you also need a yahoo account if you do it this way.