Child who wont stop hitting

I have a full time 2 year old i watch and he has been with me for almost a month now. This past week he has started hitting all the kids, even if they arent playing together. He will just walk up to any random kid and hit them really hard (most of the time with a toy). I have tried talking to him, being stern, putting him in time outs but nothing seems to work. Any advice would be so much appreciated as I am at a loss.

Hopefully it's just a stage but my advice is always the same in these instances - talk to the parents and make sure they are stopping it at home. Ask them what they do and what words they use when he does it at home and make sure you are doing the same thing consistently to stop the unacceptable behaviour.