What is our rights and what do we do on our neighbor who constantly throws things in our backyard?

We live next door to a 19 year old mentally unstable male. His parents make many medical excuses of autism, bipolar, etc. The boy constantly throws items into our backyard over our 6 foot privacy fence. He is violent and has many episodes of rage, hitting his father, workers, etc. We always throw the footballs and other harmless things back over the fence many times each day. Today he threw a tire rim into our backyard. Luckily there wasn't one of our family members or pets in our backyard, as they could have been severely injured (I was babysitting my 1 year old today). I yelled over to the father of the boy and he immediately said he wasn't home all day and kept repeating his excuse. I should have called the police, but was so angry - I had a discussion with the father in which he told me I should count my blessings. They never take responsibility in these matters. I told him I do count my blessings and feel for what they have to deal with. I told him his son is violent and needs help. He said you can't just lock him up. I said he is going to hurt someone and I witnessed his altercation on Sunday while they physically fought and the dad drove his son into our wooden fence trying to get him under control. I am at a loss. We aren't one to call the police for every little thing, but I feel we are in danger with such a violent person living next door to us with parents who are probably don't want to lose the government assistance they are receiving, as neither of them work. Any advice would be greatly appreciated.