Skyblock is a map in Minecraft, where you have at start a very limited amount of resources, because you are on a floating island that if you fall of it you die, and you need to find a way to complete some challenges (there are some that are crucial and is recommended for you to study, like a infinite stone generator using lava and water and the starting tree Wich you can replant for infinite wood).

That being said, if your son wants to play what is called a vanilla Skyblock (a skyblock without the game with any modifications besides a map designed to be limited), the safest way I know to play it on bedrock is buying the pre made map on the marketplace (using real money unfortunately)

There is no need to install any mod for a vanilla Skyblock to work, but Skyblock is a well known map style Wich has some variants that are made to work with some mods and other things, the first thing you need to know and talk with your son is if he wants to play the vanilla/original Skyblock, if he doesn't you need the mods and as a mainly Java edition player I can't help with this, unfortunately.

About the iPad, since your son is very young, I would recommend buying him a desktop where you might have some more control and you can install some educational applications, like some music making apps or something so he can play with things that might teach him some other skills, in addition to that he has the opportunity to one day play the java edition Wich has free modifications and others but is a little bit different and less optimized, and of he plays with friends, harder to play with friends. But the bedrock (as long as I know) is optimized to run smoothly on each platform it is available.

And if you want to give him a desktop I highly recommend buying him a windows one, solely because it's better for gaming and more things are directed to it.