Some toddlers are different then others. Some toddlers will stop wetting the bed once they turn 5 years old. While other kids will continue to wet the bed up until 15 years old or even longer.
If your child will wear the diaper then you should let him wear the diaper to keep his bed from getting wet. Wet diapers are much better than sleeping in a cold wet bed.
Once your toddler is dry at night then they can stop wearing the diapers to bed.