I need medical advice for my son...?

Natasha Richardson had a ski-ing accident, and I don't think a sippy cup thrown by a child would have much power behind it. Any scalp wound always bleeds much more than any other part of the body, and therefore seems much worse than it is. If your son complains of headaches, or if he becomes sleepy and drowsy (not like at bedtime or naptime but at a time when he would normally be active) then that can be a cause for concern. But basically I think you are over-reacting, the skull is a pretty thick piece of bone.