Playing rough

Lately, Holden has been playing really rough with DH & I. He has been nipping and jumping on us. He is definitely playing but I don't like the nipping -- any suggestions for how to stop it? I give him a toy, say "STOP" firmly and he stops for a min then does it again as soon as we play. He turned 1 last month so I don't think it's a puppy thing. TIA