I know it sounds really weird but can you maybe have the other kids use the potty and take a picture of their pee and poop in the potty he will use? Without the kids seeing, last thing you need is them seeing you take pictures of poop lol. But having pictures of sitting on the potty and then having pictures of something IN the potty. Make a photo board with the sequence. Point to it as he progresses though the steps. It seems odd to us, but to a child with delays being very very clear and concrete is best. So a picture of fake pee and poop in a potty may not be as informative.
It is still a long shot, you've mentioned he watches others and knows what it's all about so it might not be the most useful tool. BUT, there's a chance. The peeing doll would be helpful too. As the doll pees point to the picture chart and verbalize the steps the doll is doing.
Good luck!!