Marble jars are effective for this age child ... get a small jar for a child of this age though! Give him a set amount of marbles at predetermined times of the day (ie before lunch, before bedtime, etc.) for when there are NO toys/clothes on the floor and then take away a marble for each item that is on the floor. When the jar gets filled, you can do something fun together (like going to the park or for an ice cream cone or ???). Eventually, you'd phase out the jar but by then, it should hopefully be a habit for him to clean up at specific times of the day.

My friend used to pick up any toys that were on the floor at the end of the day and put them in a box and give them to the shelter or thrift store. That was REALLY effective!

It will get better... A lot of it has to do with being a busy little boy who probably doesn't want to stop what he is doing!