J.K.
If you cant be nice, then you cant play with us. Thats what I tell my three year old. She then is immediately removed from the room and the door shut or the gate put up. If you are in a communal area of the house then put him in his room. If he wont stay in on his own, put him in and shut the door. Kicking a baby that is laying on the floor is major unacceptable behavior. I tell my daughter to stay out and think about it until she is ready to be nice. Then she has to hug or kiss baby and tell him shes sorry. After a few times of that she was much nicer. Now she only gets sent out for being too wild or throwing toys.
Im sure he just wants attention and is not a mean child. With my dd, once a week or so I let her stay up after the baby goes to bed and we make popcorn and watch a movie or show together. That seems to help.