Your son could be a bed wetter and has bought goodnites to wear without telling you.
Or your son could be a diaper lover and just loves the goodnites to wear.
I would speak with him calmly and ask him if he's having a wetting problem and explain to him that it's okay and that he can wear the goodnites.
Or even better if he'll wear actual diapers then those would be even better.
But it's most likely your son is a bed wetter and is too embarrassed to let you know about it.
But then again he could also be a diaper lover as well.