My “daughter” identifies as “masc”?

I (39F) have a “daughter” (15 masc, 15M?) who I guess is now my son. He identifies as masc, which I guess is masculine? So I guess I will use he/him pronouns. And he prefers to be called James or Jimmy. Jimmy has talked about wanting to be a boy before, but he has been very vocal about it as of recent and at first I was just thinking it was a tomboy situation but now that he identifies as masc, I was thinking of how I can support Jimmy and get used to it.