Unfortunately, flowcode doesn't support raspberry pi, as raspberry pi is more meant to be a computer than a microcontroller.
For Rasberry Pi starters I'd recommend learning python, or if the language should be graphical, I'd take look at the scratch language. I have no idea does it have any way to interface with the I/O of raspi like the python does, as far I know scratch is more for games and interactive stories for kids to make...
For python camp (as I use it almost every day), I'd like to say that it is pretty easy to learn but can be some times hard to debug, but led blinking and etc are pretty easy to do.
For starting electronics hobby, i would recommend getting a small development board like Arduino/genuino uno starter kit (basic leds, resistors (fixed and variable), capacitors, transistors, switches, LCD display, ldr sensor etc...) I have couple of easy projects that even don't need any more than couple transistors, one led and some resistors,and internet is full of em.
To get use for the raspi, teach youself and your son some scratch programming (scratch is the funny kitten image on the raspi gui desktop), or chart drawings to get started with python...
For raspberry pi I/O breakout, i personally use the adafruit breakout board.
Remember it is 3,3V io!
Happy computing!
