You're right - you have to protect that baby! First of all, focus your attention on PREVENTION. Since you can see that he gets distressed when you are intensely involved with the baby, try giving him attention even while you take care of the baby. For instance, you can talk directly to him while you're changing the baby ("See how Mommy changes the baby? She likes getting cleaned up...see how she kicks her feet...she's a funny baby, isn't she?" and so on). As long as you give him some attention - even indirect attention - he will be less frantic and unsettled. If you have given him attention but he still manages to hurt the baby, gently get up and move yourself and the baby. If she is crying, tell him "The baby is crying now. She is hurt. We have to look after her. You have to be gentle with her. Do you want to pat her gently? She likes that.") Then guide him to gently touch the baby so you can give him lots of positive attention for being gentle. Be patient, this phase will pass eventually.