Yeah the cobra is a badass bike, and if you were on a nationl level to where the sponsorship was there to help pay for it would be even better. I have read on those bikes though they are having trouble with them and there have been quite a few DNF's with them in the gncc races.
For you gettin into the racing scene I would opt toward a Kasea or Eton. I don't know the rules out there or if you have set a cc size that you don't want to go over with your son for racing but the 50cc bikes with can be converted once he is comfortable with the racing to 70cc and both bikes can be modded to the moon if you want to.
I really wish now in hindsight that I would have picked up a Kasea or Eton 90 for my son instead of the LT80 for the parts issue and also the big shops have kind of put the lt80 on the back burner for modding and such.
I am kind of in the same boat as you on the bike deal as my son can ride the crap out of the lt80 actually he can over ride the 80 and is really ready for a Blaster, but here he cannot race the Blaster as he is to young. He loves the racing part so I guess the 80 will stay for a while longer and engine work performed.
Sorry for rambling...lol and hope this helps, Chris