I have experience of being around someone with an official diagnosis of dpd.
Your dp downplaying it somewhat. There is a core or host personality, which is constant for a period of time, but not necessarily permanent. This is likely who you are falling in love with.
Then there are a number of "Alters". They have their own names, clothes, likes, dislikes and personalities. The differences can be extreme. One of his Alters may not like you, or protest the relationship, or you may well not like them. This is hugely stressful.
He may not be aware of all his Alters, they may well hide stuff from him. But I would ask about them, names, likes, potential dangers, and if they ALL LIKE YOU. He probably has at least one child like Alter....he really should speak with you about it in depth.
I'm happy to talk about this more over pm if you think I can help.
On the plus side, you may well like his Alters. None of them are necessarily crazy or bad. He cannot help this, if he has a diagnosis it isn't put on or something controllable. There are no drug treatments, and integration through therapy is rarely successful, but he may well still be lovely, even with this problem.