Toileting issues

My 6 year old son has still yet to use the toilet for his no 2's. He has no trouble using the toilet for his wee's (still wears a pull up at night as he isn't quite there yet). He has a good understanding of most things however it seems he just doesn't want to try to poo on the toilet. 

I have tried a lot of different methods and was just wondering if any other parents have had success stories regarding this. 

I am also quite sure that his bowel movements take place when he is asleep - 5-6am. Is there a way to re-train his bowel movements to make him do it later on in the day so we can try to get him to use the toilet. I feel like when he sits on the toilet because he is used to doing it so early in the morning there is nothing there to come out and he doesn't quite grasp what he needs to do! 

Any advise would be appreciated.