He cannot find items because when he looks for one thing, his visual perception is distracted by all the items that he is looking at and he cannot filter out the other items. If he never puts an item away, it should be where he left it and much easier to find. Designate an area or room in the house where he can lay out his toys in complicated designs and leave them up for days at a time. For example, we have a large table in the kids playroom where my son will build complicated lego cities, or a city of blocks, etc. I have found that my son will put the lego away when he wants to move onto a different project.
Laundry baskets holding toys in every room works well when the child is young.
Toys brought from upstairs can be put back upstairs at the end of the day easily. Laundry baskets or boxes with file folders, large recycled envelopes in them can help sort the paper out. My son likes to save all his paper art, and we have several boxes stored in the basement. You never know, he may resurrect an idea one day and become a famous video game maker. Organize clothing into drawers or cubbyholes, matching tops, bottoms, socks and underpants.