Hi Bihdegit and welcome to Adisc.
Lots of pointers.
1, does he want you to wear nappies or does he want you to be his mummy?
2, If he wants you to wear does he intend making it a part of your intimate times together?
3, If he wants you to be mummy, does he want to use his nappies to either wet or soil in them and does he expect you to clean and change him.
4, If he does want to be a baby or toddler does he expect to be bottle or spoon fed, does he want to suckle?
5, Does he want to dress as a baby or toddler, use a bottle or sippy cup, a dummy (pacifier) etc.
6, if he wants to be a baby does he also expect sex whilst in his nappy (rubbing) or is he going to be a completely Non-sexual baby.
So you need to sit him down over coffee and cake and talk to him about his wants and desires and what he expects from you. You also have to be open and honest with him, if there is something you don't want to try that he mentions then you need to tell him so, especially when it comes to changing messy nappies (Cleaning and changing a soiled adult nappy is not like cleaning up a real baby).