Be very careful putting him on your policy; the rules have changed on this in the last couple of years. In fact, from memory when I tried to do it in my Dad's name with me as an extra driver I was told that they would not add a young (under 25) driver to anyone's policy and that it would *have* to be in my name with my father as the extra driver.
I was insured with Volksworld Insurance (HIC) last year as they could give me the best deal. As with all insurers you have to ring around for the best deal. Get some speculative quotes and let us know what you find out.
P.S. with a young driver it seems best to have a mostly OG camper as all modifications (lowering, alloys, bigger engines, etc) either make for a higher premium or companies not willing to cover.
Lame.