toilet trouble

hi there, my little boy is 4 years old and has been having this trouble now for nearly 2years!! he is fully toilet trained in regards to weeing on the toilet but just wont poo on the toilet! we have had some success occasionally usimg bribes etc but the general problem is that when he needs a poo his initial reaction is to hold it in! he has become quite ill from this in the past and we have tried EVERYTHING no matter what health visitors/doctors/teachers/friends say weve tried but its not working! After christmas we had a 4week stint of him having zero accidents and going to the toilet and then all of a sudden hes reverted back into his old ways!!!!!!!
please can you help as im at the end of my tether with it!!!!!!
thank you

[Kevin Kennedy, Ph.D.]

Examine how you were handling things during that period of success and return to whatever that was. Overall continue to be patient and supportive, never punitive, and employ a reward system for whenver he is using the potty. This will be fine - he's on the verge of completing this developmental task.