I have his birthday as May 18, 1941. He can't be more than 10 in the photos. Looking at the 1950 catalog, the fenders of most of them have the llittle chevrons and painted head tube. As GTs58 pointed out, in the 1951 catalog, the 51 model D-24 Spitfire looks like the best candidate.
The shadows in the side view photo are pretty short so I would say the photos were taken late Spring early Summer? Maybe around his birthday just as he turned 10 in May 51.