Use a water proof bed pad, directly under him.
Thereby making it easier on you, for cleaning up...the bedding.
I have 4 of them that I got from Amazon.
Works like a charm.
I put them directly under my kids, when they go to bed.
Next, full night time dryness, does not occur until even 7 years old, per our Pediatrician.
This is normal.
Boys can be later.
It is a BIOLOGICAL attainment... their nerves/brain/bladder/hormone connections and capacity.
Its not 'his' fault.
Not even some adults can not pee, all night and have to get up.
For a mere 4 year old, they cannot always, yet, at-will... wake up to go pee... he is still very young.
It has NOTHING to do with daytime pottying ability.
These are 2 separate, mastery stages.
My son, is almost 4.... and he is in night time diapers, and is not dry all night. My daughter, was already 5 years old when SHE was able to then use underwear at night... and on her own, 'recognize' the need to wake up in the middle of the night to go pee.
Per my Daughter's Teachers from Preschool and Kindergarten and 1st grade... these things are normal and happens... but they said the parents will not admit it... but will tell her, the Teacher. And some kids, still do have daytime pee accidents still, at these ages. Normal.
all the best,
Susan