There is no misunderstanding. You are offering the time. He either uses it or he doesnt. If he doesnt show up on any given day after agreeing to use the time - send him notice in writing that he must either pick up DS as planned or inform you personally that he is not able to so that you can be certain to do so, if he can't do that, you'll have to make other arrangements for DS' care.
All of the melodrama in the middle isn't necessary. Offer the time - do it in writing to prove you offered it - and move on w/ your day to day parenting.
Check out "Joint Custody w/ a Jerk", you can buy it on amazon for around $10.00. My x goes through times where he is intent on making everything as difficult as possible - and this book saved my sanity! Maybe you can benefit from it too..
Good Luck.