you are an amazing person for being very supportive of him. thats one thing he needs and your loving.
It is very hard for some people in the ABDL community to open to our partner fearing of being rejected, made fun of, and being told by their whole family and friends. It will take time for him to open up to you and show his DL side to you. Let him take baby steps around/to you. Let him wear under his pants around you so he can let you know, "hey i'm taking steps slowly but it's progress." If you have to tell him i'll always love you no matter what, i will never stop loving you. I wish you both the best of luck