Thank you for asking about how to help your partner.
I dressed for many years before my wife found out. I would not admit to myself I was a crossdresser or anything of the sort. I was ashamed of my actions and did not want anyone to know. I did not want to know about my actions.
My wife has know for 3 years and does not understand why I dress but will not stand in the way of my happiness. So allows me to dress how I want when I want. She does not go out with me and is afraid of what will happen if people from work see me, she is afraid I will get assaulted for being me. She loves me and cares for me and does not want me hurt ever. I still feel like I need to hide this part of me sometimes. Her only ask is that I not hide anything from her. Coming to terms with who I am was very hard work, now my wife is doing the same hard work.
Long emotional discussion yesterday. Then we went out to a drag show with me in a skirt for the first time, with her.
My advice. Let him know that you care and want to be a part of his life. Your love and support without pushing him will have him come out of his shell.