S.H.
Night time dryness is not something that is BIOLOGICALLY attained, until even 7 years old, and is completely normal.
It has to do with their physiological development... not them being naughty or lazy.
Just use a waterproof bed pad directly under him. That is what I do. My kids are 4 & 8 years old. I have 4 of them that I just rotate, if need be. Makes clean up real easy. I got it from Amazon.
My daughter was already 5 years old, still using night time diapers... (pull-ups are useless because they do not hold much and leak and are expensive). THEN one day, SHE told us, she was 'ready' for just wearing underwear at night. She was right.
I do NOT wake up my kids at night just to go potty. I rather them sleep. I do not restrict liquids at night either.
My son is 4, and still wears night time diapers and for naps. He is not dry.
Bear in mind, even if dry at night... there WILL be times when a child DOES have a pee accident in bed. It is normal. It is childhood.
My daughter, even when 7 years old, had pee accidents sometimes. Once, she even had a dream that she WAS on the toilet, until she felt herself wet. NO biggie. I just change the waterproof bed pad, she changes pj's and back to bed. I put the bedpad DIRECTLY under my kids, not under the fitted sheet. The sheets never gets soiled that way.