Diapers would be better but he might not like the designs that Pampers or Huggies has on their diapers.
You can get some diapers through honest diapers that might fit him that are plain white diapers without any prints on them.
I've heard pull ups tend to leak if your son is a heavy wetter at night so it's best to have him wear regular diapers that are more absorbent than pull ups.