Brendan is getting really violent, especially towards Tristan. This afternoon he pushed Tristan into the toy chest (wooden and very heavy!) and closed the lid on his head. Tristan has a split lip, blood nose, cut on his forehead and a whopping bruise in two places on his head

This kind of thing is happening EVERY afternoon. What am I supposed to do? How do we stop this

Smacking doesn't work for us (nor is it something I want to do....)
Time out doesn't work, it makes Brendan worse.

He doesn't really understand much else as far as punishment....

Please help