Yes you are on the right track. The International Journal of Impotence Research (2003) also states the average erect penis to be 12.9–14.1cm long :)
In an Italian study published in 2001 of 3,300 soldiers age 17-19, Ponchietti and his colleagues found the average stretched penis length (this is very close in length to the erect penis) to be 12.5 cm. or 4.92 in.
The 6.3 was from a study done by Kinsey over the phone. I would trust the Italian soldiers study who were chosed on a random basis. Do you think the guys with 4" penises are going to volunteer that info?
2in sounds about right. The average size of a mans erect penis is 6.3in. If your son is circumcised he will naturally look smaller (but in fact wont be compared to uncircumcised). Yes you should pull back the skin to clean it, however the skin is fused to the head of the penis which can be extremely painful and takes a while to release. It needs to be done over a period of time, i probably wouldn't recommend it for a child so young. For you information i began to do this during the early teenage years.