Think From the Perspective of a 5 Year Old
I remember very well how I felt about airplanes at that time and mine was not something like an RC model. I wanted something that I could hold in my hand and "fly" with graceful paths defined by the movement of my arms - peel off, dives, low altitude straffing runs, smooth landings. It was two solid wood clothes pins interlocked. I think if you really want you son to feel proud you might consider making him a small solid balsa model with no more than a 6" wingspan. Not something clunky with no airfoil and no smooth rounded edges. The three view in your plans book should give you sufficient dimensional reference to make him something that will be deeply appreciated and enjoyed.
Then again, when I was 5 years old Pearl Harbor was attacked ... things were different then. I may be wrong about what children of today think about.
Bob Axsom