Dear Conny,
I am so relieved to hear my words managed to bring you some much needed comfort. And that you are feeling less panicked now. How both you and your son have reacted to the situation are both valid and natural responses to a very difficult diagnosis. Im glad to hear your son apears to feel safe with his doctors taking care of looking into treatment and with his familys support. I am also happy to hear that your sons life is so far relatively uneffected by his AVM and he is doing his best to carry on and do all the things he enjoys. I think for me I went into information search overdrive whist staying relatively calm initially whilst my mum went into worry/panic overdrive for a long time. For the first time in my entire life, I had a problem in my life that she had no power to fix. It is so easy to say “keep positive!” and another thing to do it. Dont criticise yourself for your valid and natural feelings.
Something I forgot to mention is, and as you know my case is incredibly rare, as my health declined pretty fast with me losing many things that gave me a sense of pride and happiness and my mental health took a big hit - I started cognitive behavioural therapy to help me cope. It didnt change my situation but it did however help give me the tools to better cope with the situation I was in. It was also really comforting to talk to a professional, impartial and non-judgemental person who was neither family nor a friend and have a safe space to be completely honest about my emotions for an hour a week.
I think this could really help you too. In future if there are any changes to how your son manages emotionally or his symptoms or if he undergoes treatment, it could help your son too. Having you already having taken your sessions or already be in the process could prove to be very helpful too in future 
If you are unable to access in person CBT sessions although they are the best recommendation, I have also made a post with some free online resources recommended by my therapist: Self help online sources
Always feel welcome ofcourse to reach out and vent or share your concerns or ask for advice or experiences here. We are all here to help support you on this journey.
Lots of love and best wishes for you and your son and family,
Corrine