There is NO way You gather the Legos.
Take the son by hand, lead him to the Lego, show the piece,
ask to pick it up, walk together to the place they are supposed to be and say if there is one more not in place, then there is no TV today (or whatever he likes, he's missing the thing or event). If all are in place tomorrow, the regular schedule of joys resumes.
At this age, he should know the rules WORK, so be very steady with it, never back up, never, and he'll get it very soon!
With all the other disorganised situations do the same pattern. Set up the rule, and make sure you yourself follow it by taking away some joy if the rule is not followed, and resuming the right schedule if the things are well.
I wouldn't suggest treats for doing things right as then you will soon run out of money buying him candy as he'll figure whenever he is doing all well, he is supposed to get a candy.
Good luck, don't be upset, do it with a smile "sorry, sunny-son, this is the rule, I am sad you have no cartoons today, but you can help us out so tomorrow we will both watch a movie together, you're the Master of the situation, so please go for it!"
Good luck!!!