Perhaps he is'nt stimulated enough, you say they have toys at the nursary do they also play games, read to the children ,is there an outside area that they can play and climb and use up their Physical energy, .How many children are in the 2-4 room, is it possible other children are acting out and he is copying them. Check if there is anything at home has changed that could have upset him aswell, new Baby? new home . Family.