7 month baby purposely banging head?

I babysit my sisters baby. She used drugs with him when pregnant, but her baby turned out healthy. However her baby does seem to be much harder to handle than my baby who is almost the same age. My sisters baby seems to throw what looks like tantrums already! is this normal? Today he was purposely was banging his head again the table leg. This is where I am drawing the line. i know my sister gets frustrated so I am really hoping this head banging is not an early sign of child abuse.