Is he holding it or is he constipated? You can definitely put him back in pull ups for now. My DD wasn't fully out of pull ups until she was 4 so he has time.
It can lead to all kinds of complications if a child starts to hold his bowel movements. Can he go to nursery in pullups? It might ease his tension. 3 years old is pretty young to expect much from in the potty training department, especially only after three weeks of trying it.