I would have done the undies under the diaper thing too.
I'd also stop calling them "big boy undies" or anything like that, because of course he wants to be a big boy, and then wearing a diaper will make him feel like a baby, particularly now that there is a tiny baby for him to compare himself to. He can't control when his bladder will keep him dry all night, and I wouldn't relate that to being a big boy. Maybe tell him that lots of big boys still have to wear diapers at night, for a long time after they're able to stay dry during the day.
Another thought - maybe buy a different style of diaper, pullups or good nights or something - and say that those are "big boy nighttime diapers" as opposed to baby diapers.
I'd also stop calling them "big boy undies" or anything like that, because of course he wants to be a big boy, and then wearing a diaper will make him feel like a baby, particularly now that there is a tiny baby for him to compare himself to. He can't control when his bladder will keep him dry all night, and I wouldn't relate that to being a big boy. Maybe tell him that lots of big boys still have to wear diapers at night, for a long time after they're able to stay dry during the day.
Another thought - maybe buy a different style of diaper, pullups or good nights or something - and say that those are "big boy nighttime diapers" as opposed to baby diapers.







