So sorry you're going through this OP
I have a friend who is recently bereaved, very stressed, and struggling with lockdown as the next door neighbours have children who get up early, play musical instruments badly in the house then go out to the garden where they scream all day long whilst bouncing around on a trampoline.
These kinds of situations are really difficult for all concerned.
Does your neighbour know about your child's special needs?
Were you on speaking terms with your neighbour before lockdown?
It seems to me that there are layers of stuff going on here.
I can understand how your neighbour might feel very upset because, during a time when we are meant to be social distancing, balls keep flying into her garden. While this might have been annoying at the best of times, these days the consequences could be extremely serious, even life threatening, especially if one has ever hit her in the face.
She cannot escape the screaming and her garden, that should be her refuge, is turned into a loud, unpleasant place where she might get hit on the head with a ball at any moment.
You are doing so well struggling with your son. You cannot help his behaviour and the lockdown can't help. It must be very hurtful for your neighbour to burst the balls. I can see how it might feel like an agressive act.
And this is where the layers of hurt come into it.
Your son isn't trying to deprive your neighbour of her peaceful refuge. He is not trying to hit her with a ball. However, especially if stressed and deprived of her usual restorative garden experience, your son's activities may very well feel aggressive to her.
I can also understand how your neighbour popping the balls can feel very aggressive to you.
Is there any way that you can let her know that you don't wish her any harm, you would stop your son from screaming if you could and that you will do your best to stop the balls flying into her garden?
Because beyond the reality of what is happening that both of you have limited control over, is probably a feeling on both sides that you feel agressive towards one another. The reality probably is that you either don't or didn't to start with but that aggressive feelings can arise out of feeling anxious and under attack.
I think that making an effort to show that you care, and your neighbour needs to do likewise, might possibly help to lower the temperature of the situation?
I'm suggesting that when your son screams your neighbour doesn't just hear screaming she hears "I don't care about your enjoyment of your garden".
When the balls fly over the wall they may be perceived as a similar communication.
When she pops the balls, which she may be doing out of ignorance and fear, it probably feels to you as though she is trying to hurt you your son.
I am probably not expressing myself very well but does that make any sense at all?