could this be his way of trying to be top dog of the house , i had similar with my 1 female mastiff she would bite over food treats toys and then started trying it with me , shes been put in her place sinse and the behaviours vanished but i know how scarey it is when you have little ones around , its always best to have 1 person top of the house in the dogs eyes as i think it can confuse them .
hope your able to fix things x