Homeschooling

It might help to give both boys a regularly scheduled block of time with you, and give them a way to track how much time is left before it's the others' turn. If they know that every day from 3-4 you are with this one, and every day from 4-5 you are with the other, and they can read a clock to watch when it's getting closer to their time, that will help establish a routine.

You haven't said how long you're expecting him to entertain himself, but you might need to do small chunks interspersed with independent work for the older one, so the younger one isn't on his own too long.

I don't know if incorporating him as a sidekick into his brother's activities will be fully successful - he might sense that these things are not about him. Giving him his own school projects might help him feel equally important.