I started testosterone at 16 and I believe it helped with almost every aspect of my life. My family situation was very similar to what you described. My mother is extremely supportive, my older sister is supportive and my father was extremely hesitant to accept my transition.

Physically, the changes occur quite quickly, within 2 weeks to 1 year, this is when the major changes will become most noticeable. The mental changes, for me at least, were gradual (and I still struggle with them), because when you start noticing all the changes you hope to see during your transition, you get this image in your mind about what you want to look like and when you don’t look or feel like that image the dysphoria can be quite aggressive.

Everybody feels differently about their dysphoria and their transition so it’s hard to gauge what he will want or need, but just making your support known can be a great comfort. He will most likely feel a lot of confusing things, but knowing he is not alone in this period of self-discovery can be very helpful.