Hi Mike;
Great videos, and glad to see you both have the patience to stick with it. My son Sean was only 3 years old when he soloed on a Goldberg Wizzard, and I know I have video of that around someplace, but can't seem to find it. Sean was pretty much raised at the flying field and been around airplanes of some sort all his life. He's 28 years old now, and a U.S.Army UAV operator with two tours of Iraq and one in Afghanistan under his belt, and recently moved up to the Army's version of the Preditor, called the Grey Eagle. He checks in here on Stunt Hanger from time to time. Your boy is 4 years old now, but from here on out there is no telling where he could go. Get as many pictures and as much video of the journey as you can, because these are golden times for you and will be fun to look back on with him years from now.
While he is this young, keep him in the small stuff until his hands get a bit bigger. The .15 size trainer will pull a bit more than he may be able to hold on to. Sullivan makes a handle that has 2" and 4" line spacing on it, and the grip on it is smaller than typical control line handles, or make him something custom. A model in the .15 size range will pull about 15 pounds or so, and you can simulate that with a fish scale rigged up to the handle to give him am idea of what to expect. The larger size, louder engine, and longer lines may intimidate him a bit, but just remind him that if he can fly the small one, the bigger ones are easier. If you have some .049 models that will fly out at 35 to 40 feet lines, that will give him some idea what to expect also. The Flight streak Trainer will fly around 52 to 60 feet. In my opinion, he's not too young to learn how to start and handle the engines either. HE may nick a finger now and then, but the .049's are pretty tame, and won't do much damage, and that is how respect for the prop is learned. Afterall, we weren't much older than that when we started cranking them!
So congrats to you and your son, and thanks for posting the video. It brought back lots of memories for me!
Good luck and have fun,
Dan McEntee