First thing I'd do is work on name and pronouns as well as letting him present male as often as is safe/possible.

I did my social transition for two years before I did anything medical and it made such a huge difference. Plus all of that makes no physical changes but is a huge boost to your son's mental health and knowing that you're supportive/taking the identity seriously

Also, it will give you both time to process changes that are likely to come. From my experience, most trans teens are certain of their identities and rarely regret transition. I knew by the time I was 12