Daytime pottying and night time dryness... is NOT the same thing.
It is 2 different, processes.
Night time dryness, is a PHYSIOLOGICAL, occurrence.
And it can take even until 7 years old, for it to develop.
It is about the body's organ development/brain connections/myelin nerve sheath development, etc.
You cannot speed up, organ development.
At night, per both my kids, I used Huggies night time diapers. NOT pull-ups. Pull ups leak and don't hold much unless you get night time versions of it.
AND I have, 4 waterproof bed pads, to rotate and put under my kids at night. Then the sheets do not get soiled. I got them from Amazon.
IF my kids had an accident at night. So what. I just changed out the bed pad, put a new one under them, they changed their pajamas and went back to bed. No biggie.
You NEED to get, some waterproof bed pads.
Your son is young. My son is 5 and still wet at night.
My daughter was too at 5 years old and even at 7 years old, she had night time accidents.
Normal.
You are not wasting money on pull ups. His body and organs, physiologically, are not developed yet at the stage, to be dry at night.
For my son, even at night, his diaper can be soaked. I change him 1 time a night... to prevent an over loaded diaper. And sure, he has a waterproof bed pad, directly under him.
ALL kids, have accidents. Day time or night time. It is childhood.
Of all ages.
EVEN all of my daughter's Preschool/Kindergarten and 1st Grade Teachers, all said the same thing.
My kids.... NEVER EVER, got "confused" about wearing a night diaper at night, versus underwear at day. Because... I simply EXPLAINED to them, about their body and its development. And it never ever, made them relapse or backtrack or regress in their pottying per daytime.