First off, welcome to the forums! You definitely came to the right place, and it's always great to see supportive loved ones looking to inform themselves for their S.O., friend, family member, etc. So go ahead and give yourself a pat on the back for that!
Now, on to your question: testosterone affects all men differently, and I guess the best way of thinking about going on T is that it's a second puberty for trans men; we might get moody, but no more than any other teenage boy. As far as I know, T does not cause the "roid rage" typically thought of when people hear that someone is taking testosterone, because the goal of Hormone Replacement Therapy is to get the patient into a normal male range of testosterone for his age. Now, a little pubescent, hormonal behaviour paired with an unsupportive/verbally abusive altercation with his mother? That might cause a storm or two, but it shouldn't change how strongly he feels about his family, friends, or even yourself. We don't love any less on T, in fact you may find that he finds himself even more at peace around you and can give you more of himself, since some of his internal struggles will hopefully be alleviated with his body running on the right "fuel," so to speak.
Another thought: if you have concerns, voicing them might not be the worst idea. That way, he can actively watch himself for behaviour he finds odd since starting T and can correct it as needed; it's something I've tried to be very aware of doing, even if only in hindsight in some situations. Retroactively correcting behaviour is way better than letting stuff slide and severing ties you're going to hit yourself for later, right?
Good luck, I wish you the best!
