Hi there-
I agree that this is something to bring to the doctor's attention. You might even try to call the nurse this morning to at least notify the doctor of what's going on. From the dr's perspective, this may need to be addressed before next week. I just don't know. Let the doctor make that decision; better to be on the safe side.
Try to think back to the meals he had on the three occasions this happened. Maybe there is some kind of allergy or reaction to a certain food....???
Just like your son, the food is still recognizable the few times my daughter has thrown up in the middle of the night. Just FYI.
I would still call the dr. this AM. Good luck and let us know what happened. I'm stumped.