How can i stop my children fighting at bedtime?

I have a 2 year old son and 3 1/2 year old daughter who share a bedroom. Every night without fail, My son starts being nasty to my daughter when theyre put down to bed. He hits her, jumps on her, shouts at her and 9 times out of ten in results in tears! (mine too!) I have tried bein firm with him i have lost the rag with him i have tried distracting them from what was going on before i went in i have tried putting him to bed an hour before my daughter, so he's asleep before she goes to her bed, but nothings working. We are all exhausted the next day from the bedtime battles with them. Ive tried keeping him awake during the day so hes extra tired at night and just falls asleep but hes very active and falls asleep in the car/pushchair or just in the afternoon in the house so by bedtime hes not as tired as i would of hoped. I am at the end of my tether and struggling! Any advice/suggestions?? Thank you