Great stuff, brother. You know how like talents and interest, often skip a generation. You really have something to build on there, with that kind of interest. Honestly my first triumphant repair was a Merc Super 10 that my Dad was gonna drop in the lake after having multiple repair failures. He was having trouble with fuel contamination from a dirty fuel tank. He had even hauled it back from our portage lake in remote NW Ontario, after it ruined the trip....for denying us access to several limits of ice cold spring Walleye. At 8 years old, I wondered when he dismantled 12 ga. shells only to remove the pellets and attempt to clean the inside of the tank by shaking it. He never did get it clean enough to be reliable. So as he was loosening the transom clamps in about 30 feet of water, just about 10 seconds after his wrist watch broke off and flew in the lake....on the 40th pull of the starter cord....I began to cry and plead with him to save the motor for me to try and fix someday. Ha! He looked at me intensely, then back at the motor now hanging over open water....and he paused....looked up to the sky, as if to say "what the he!! am I doing?"
He hauled that 10 all the way to our new home in Duluth Minnesota, where he built a new life for us in the TV broadcasting business....but me, having an opportunity to take over part of the basement and start repairing outboard motors. I repaired and sold that Merc for a handsome profit and it made a permanent impression on Dad. He knew that I was cut for mechanics, just as his own Dad was.
Keep encouraging that little fella, you will be among the world's most treasured Grandpas. Yes, go ahead and build that little hydro....and remember HE'S building it, your just supervising.