Night Time Dryness... is not something that is biologically attained, until even 7 years old. And is normal. Some are later.
My son is 5 and still in night time diapers.
My daughter when 5 was in night time diapers as well.
I do not withhold liquids at night.
I do not wake them from sleep just to go pee.
I rather them not get lack of sleep, just to be woken up for peeing, from me.
Your son is NORMAL.
You cannot 'train' for night time.
Night time dryness is not the same as, day time pottying.
It is TOTALLY different. Not the same at all.
I have, for my kids, waterproof bed pads to put under them.
THEN that makes leaks, much easier to clean up because then the sheets do not get soiled.
I have 4 of them that I rotate, and I got it from Amazon.
My kids do not and did not... get 'confused' about night time diapers but not for daytime. Because, I simply explained to them that their body is still developing and not ready to be dry at night, yet.
They understand.
Night time dryness... is about ORGAN development/brain & bladder connections and nerve myelin sheath development. Not about age.
AND even if daytime and night-time pottying is mastered, pee accidents DO happen still.
My daughter at 7 years old, still had had pee accidents at night.
ALL of her Teachers, from Preschool to Kindergarten to 1st Grade... ALL Unanimously said... this is NORMAL. Kids these ages, wear night diapers. Accidents happen. They are young. It is normal... but the parents won't admit it and feel embarrassed.
But they all said, it is normal.