Toddler Constipation- what can i do about it?

My son is three years old and suffers from constipation on a regular basis. I've tried persuading him to go voluntarily, offering him rewards, feeding him fiber rich foods, fiber gummies, and i was using liquid suppositories when he wouldnt go and it had been too long since he last went. I don't know what to do and he is scared to go. Right now its been a week and 1 day since he last went and i am worried. What can i do!! I dont want to have to continue giving him suppositories- they severly distress him. Thanks