Sore chickens

Hello all, has been a long time since i posted and received such good advice about my chicks (which sadly were both boys so couldnt keep).
I'd like some more advice please. I've noticed lately that Dexter our cockerel is getting quite rough when mounting the ladies. A couple have quite sore looking patches on their backs. I cant seperate them and dont want my girls having sore backs. We only have 6 hens now with him and he picks on the same ones.