True. I was thinking more along the lines of economic SHTF were a company like Kel-Tec might fold and parts would be tough to find as they are the only maker. But, even that is pretty far fetched.
Then I would suggest a Ruger M77 or Remington 700 in .243 Win. The recoil will be manageable and the round will be potent for dear. It is a very good caliber that does not get enough attention. Also, Remington make a "youth" model Centerfire Rifle - Model 700 SPS - Remington Centerfire Rifles
There is nothing wrong with the Kel-Tec except for if you get the customization or upgrade bug you will be quite limited. Also consider parts availability if you are thinking of a SHTF type scenario. If you are going to buy it and leave it alone then you should be GTG.
What kind of money are you looking to spend? Can you hunt with 5.56 there? If so, why not an AR as it has an adjustable stock and could grow with him (physically and technically) as he gets older.
So add a $125 Bushnell TRS-25 and the price of the Double Star is now 794.00 plus FFL transfer fee of $35+ or so. Total will be $829+ which may be out of his price range.
Also, since you obviously own or have owned a new style Carbon 15 (with the dust cover and forward assist like what...
Springfield does give you a lot for your money, but I would not say that Kimber is overpriced for the fit/finish, especially compared back to Colt. Also, Para Ordinance make a very good 1911 that may even save you a few dollars.
My cop friend who is a firearms instructor for his PD: I would advise against using that type of grip in any real world situation.
My FIL who is an ex-marine and served in Vietnam: WTF? Why would you risk loosing control of your weapon with such a grip?