Ouch! Legos are the WORST to step on with bare feet!
Try implementing some behavior modification strategies to start learning to pick up after himself. And, whatever you do, don’t pick up after him yourself. Call him back, and ask him where he thinks the lego on the floor should go, and ask him to take it there.
Here’s the behavior modification approach I used when my son was hitting, but it can be used on any behavior:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism