Amazing! Such positive news! Just keep plugging away at the light, fun, contact. Don’t let anyone paint you as the villain.
If it were me I would perhaps invite DS to the sessions and make it clear it’s an invitation, that you are inviting him because he is important in family and thus shouldn’t be shut out of any family activity.
I would also make it clear that you have no expectation for him to accept that invitation and that you trust him to decide his own priority re: not missing school and re: whether this is something he is interested in participating in.
Also that if he does attend once, there is no obligation for him to go again.
I agree with others re: making a fuss of him and a cinema trip or similar shared activity likely being more valuable than attending family therapy.
It might be that as you go to more sessions you come up with some topics or questions you can ask DS about yourself and then bring up at the sessions.
My DH has always found long car journeys a good time to talk to a teen - you are close together, in a private space, but not looking directly at each other. No one can leave the ‘room’ until the destination is reached but it’s an absolutely fine environment for just sitting together without having to constantly converse.
Plus you can put some fave snacks in the glove compartment and let your teen choose the in car music, which gives you an in-road to asking about music/podcasts they like etc.
Once upon a time this stuff was obvious because teens played their music on turntable or CD players and everyone could hear. Nowadays they are often plugged into a device via headphones, separating them from full integration with family life.
Re: the name, you could offer a compromise with an initial (or a combo of old and new initials, if that gives a name-like result, eg AJ) or a gender neutral nickname based on old or new name. See if you can find a way to meet in the middle before going all the way to new name, but probe gently.
You could try a good cop/bad cop variation, something more along the lines of ‘understanding cop/bewildered cop’ where you ask gentle questions using the ruse ‘can you help to understand [thing] better so that I can explain it to dad? He’s so sad and confused and I don’t know what to say to him’
I don’t think you are quite at the right time to try that yet, but keep it in your back pocket!
Oh, and ask her to send photos from the concert!