Update:  I spoke with the principal last night and he apologized and set up a meeting with the school liasan (cop) this morning. My neighbor also told me that her son told her this same  kid was threatening her son and was showing him pictures of guns from his house on his phone. Thank you for your input I'm very hopeful the cop will do something. 

Update #2

My husband and I met with the school cop. She reviewed the tape of the bus and said she couldn't be sure it was intentional because the other student wasn't specifically looking at my son when he through it. She then said it was inappropriate that my husband got on the bus after a kid told him our son was hurt and asked the driver "what the hell happened" the cop said that they could file charges on my husband for scaring the kids by getting on the bus. What the heck is wrong with this world. Any parent would jump on a bus if another student ran to you saying your kid was in the bus hurt. The system is so messed up and now that violent child only received a bus suspension. I'm heartbroken for my son and my husband .