What would you do

My husband was looking after our 2.5 year old and 1 month old while i.took the dog out. When i got back my son was crying and my husband was saying something. I walked into the room and my husband said our son threw a tantrum which included throwing my husbands phone at our newborns face (he throws when he gets upset which were working on) and my husband said he instinctively bopper our son across the head. He said he apologized immediately and didnt mean to do it. We both agreed it was a completely inappropriate reaction. Whay would you do in this situation. My husbands never even raised his voice at our son before.