We are going through similar at the moment. We had a good few days of signing incident forms as my son was constantly hitting, pushing and grabbing other children. It’s been tricky for me to discipline at home as he doesn’t do it at home (he doesn’t have siblings) and I don’t really want him doing it when we are out and about so I stay very close to him. We just talk a lot about gentle hands etc. and gradually he seems to be getting better. I think at this age they are testing boundaries and struggle to control their immediate response. Have trust in the nursery as I think they will do their best to correct him and gradually this should start to get through to him
I would introduce thinking time at home, so he has an understanding of what it is and why he’s being asked to sit x