Honestly, he's three (almost) and should know better. He obviously acts like this at home to get his own way, and it works.
If the mom has blown off this behaviour in the past, then I don't really think you are going to get any help from home. However, I would probably write them an email/letter or have a face to face conversation about his behaviour, how it is not acceptable and if they do not work with you to rectify this behaviour in whatever time period you give them, then unfortunately, the boy can no longer stay at your daycare. He is a huge liability for you. What if he seriously hurts another child when throwing toys etc. The other parents are not going to be happy at all. I know it's a PITA to replace siblings, but maybe start advertising now during so you aren't out too much income.
I'm still trying to find a replacement for my biters, and thought today to myself that maybe I should rethink my decision to replace them as they have been doing so well lately....and lo and behold, one of them tried to bite my son this morning. It reinforced that they are a liability to me, and they need to go.
Hope everything gets resolved the way you would like. Good luck.