Keep him busy. Prepare games to play with him. Try to brush up on stuff like Power Rangers or Pokèmon. And don't treat him like a little kid. Look after him but don't treat him like a little kid.
Try to bring over some of your stuff and share.
And be that cool babysitter that every kid wants.
Hope this helps
Author's response
Good plan, I can find a load of pokemon stuff.
I shall be the awesome babysitter