Ugh...that sucks. I'm not sure you are going to be able to get rid of his morning nap so soon with his erratic sleep schedule. Is there a way you can talk to the parents about leaving him in his crib until a set time so that he learns to sleep until a better time of 7am or so?
Other than that, I think you will just have to play around with his morning naps. For the days that he's up early, let him sleep for an hour, on that days he's up later, maybe cut his nap shorter so that he'll go down with the rest of the kids in the afternoon?

Good luck!