i went to a friends home that had the dining room as a playroom and it was pretty hard keeping it clean and all the toys would spill out to the other rooms because this room was close to all the others. It looked messy too. And the first room you passed when you walked in.
move office into the dining room. Move playroom up into the office. At his age, he doesn't need too many toys. So keeping them downstairs would be fine for now. As he gets older, upstairs would be best. Why can't his bedroom also be a playroom?