Those are very popular. When we were there with my niece (without any special needs), she loved the penny machines. There used to be a list of the machines someplace, but in general, we found one in pretty much every shop.
One caution - if he
needs to collect everything - be aware that most of the machines have 2 possible designs. And be aware the sometimes the whole machine or just one design is out of service. Not real common in our experience, but it could happen.
Also, they had a penny album that you could buy in the parks. I think around $10, but very good for keeping the collection organized and (well) collected.
That would be like a social story.
I know people have written about making some sort of personal 'guidebook' for their kids with pictures of the park (look in the photo part of the
DIS site).
I have not done this, but one of the ways I had thought of to have some flexibility to it would be to treat it more like a scrapbook (with a DIS name like mousescrapper, might you be a scrapbooker?). You could have some pages with permanent parts on them (the parts you know won't change) and have some extra pieces to add for the things that might.
What I was thinking of was to print pictures of the different attractions on
sticker paper. Cut the printed sheet into the individual pictures and you have stickers and store them in a envelope or coupon sorter that you can bring with for the appropriate park you are visiting that day. As you are going on/know you will be going the attraction, your child can add the picture sticker to your schedule. If you need it to be changeable, you could cover your pages with contact paper or something that would make the stickers removeable.
If you need more flexibility than that, you could print the pictures on 3 x 5 or 4 x 6 cards and use a photo album to slide the cards in and out of as you need to change your schedule.
A plus of either of those would be that he would have a nice little start on a scrapbook of his own. After the ride or at the end of the day, you could use it to ask him about his day; write his comments on the appropriate page and he would have his own personal WDW guide for next time.