Personally if this was me, the next time the little boy is outside and throws something over, I would calmly and nicely tell him "Here you go darling but please don't throw anymore things over the fence it might hit one of us and that wont be very nice" Id say it when the mother is outside. If they then come to the fence I would then discuss my concerns with them. If they get angry Ill just make my point that i don't want it happening anymore and then just walk away. I understand the little boy has Autism, but we had 3 boys with autism next door to us and in all honesty they were very well behaved kids. The parents were very responsible though and taught them wrong from right. Autism or not they can learn. This mother is the problem she sounds like a lazy cow.