Maybe hard to answer, but itch can be within the anal canal, or on the outer skin, and felt like anal itch.
Some causes are pubic lice, herpes virus (small translucent blisters on a reddened skin), intestinal parasites (especially likely, if it's only at night, he should have a stool test for parasites), hemorrhoids, Crohn's disease...read more about
Rectal and anal itch.