Well it sounds like you have tried a lot of different things! The only thing I can really think of at this point is not trying. Put things out and if he is miserable and chooses not to be interested, then he is miserable. Some kids actually do worse when their parents are engaged and watching them and trying to interact. It's like the enjoy putting on a miserable show and having you involved. I can't explain, I just know in working with kids all day that it happens. I know you already have had him take the lead, but maybe just be in the same room but not involved at all.
Another idea is playdates with other kids. I know they may not play together, but this may give him ideas about how to play or what to play. Or just go and pick up toys and start playing with them. Don't ask him to join you, just go and do it and see what happens. Build a tent, start painting, etc.
That's the only thoughts I have, maybe others have different ideas. Good luck, hope the coffee helps
