Okay, I'm going to link to the video because it explains the mechanism well. The idea is that it communicates to the child that there are certain expectations throughout the day, some fun and some not so fun, and lets him know that if he gets through the not-fun stuff, something more fun is coming. This actually starts pretty low with strategies I would use with kids who are non-verbal communicators and obviously that doesn't apply to Josh. If I would recommend any of her schedules in the video for you/Josh, it would be the last most advanced one, but I would actually make one differently for Josh if I set out to make a schedule with him in mind. I'd probably use digital photos from a camera combined with written words underneath and have him check off as she showed at the end. You could put it in a small binder or notebook that would fit in your purse.
The idea is you are communicating a sequence of ideas in advance so he knows he has to get through certain things to get to other fun things. It doesn't mean you have to have a rigidly planned day, and it also doesn't mean you (being Mom) can't make changes to the schedule as needed. The point is he will see those changes. And when a day is different, it helps him understand how a day will be different so he doesn't freak out when he has to stop doing something he likes even though something more fun is coming.
If you don't like the schedule idea, a timer is another option. Let him know he has a certain amount of time (whatever works best for him - 1 minute, or 3, or 5) until playtime (or whatever) is finished and he has to do the next thing. Then set the time on your clock. With my kids I've had success using this both with fun things and with NOT fun things. It really helps them knowing that an experience they don't like (like work time) will be over soon. I often set a digital timer and set it out so the child can see the numbers go down and he knows when it's time to stop working.