If you join a go ride club I think kids get a free 'Silver' licence and adults a 'bronze' for the first year.
Your son wouldnt want to race experts in the 12's in his first year, novice would be best. Some of those 12 year olds in experts are nearly as fast as me, expert would only put him off. If you race novice you can still do the brits at the end of the year (providing you do enough rounds in your region to qualify), the only thing is there is no novice class at the brits, so it's in at the deep end at the bigest race of the year. I know in the South you can only do one year in novice then you have to move up to expert, I'm not sure about EA, maybe Mosschops can put me right on that?
I hope that helps a little.
Miles.