Hi My heart feels for you, I'm a mum too,  when our  children suffer pain, we do too.  I'm guessing your boy is a grown man from reading your post, you can't sit him on your knee anymore, but you can hold him and tell him you know he's scared and that it ok to be frightened, but you've got him and you will face this dreaded disease together.  Forgive me, I'm trying to imagine what I think I would say if it were my beautiful boy. Try to  be strong for him today that will help you to cope with the day too.  Know I will be thinking of you xx