I am a broker. No auto insurance companies in Ontario are allowed to credit score since the new auto reform came in on September 1, 2010. It's true, Aviva was credit scoring, had a 3 tiered company, but that's a LONG story.
Your son should be able to get a good rate if he uses the same provider as you, so that he can get multi car discount and some companies will give him multi line discount. (Provided he still lives in your household).
I'm sure he's encountered some high rates, Hamilton is really expensive compared to it's surrounding areas. It appears that Aviva may be targeting younger drivers at this time because they do have competitive rates right now. If he got his G1 @ 16 and moved through the levels to G, he should be at least a 6* driving record right now.
PM me if you have any questions!
Good Luck!