HELP!

Please help! I need advice. Kaleb is biting. Not just little nibbles, but almost blood drawing bites. It seems to happen when there's a struggle over a toy or something, and moms are out of the room. This is not good. He is a strong willed kid, very active, and a bit possesive(maybe cause he is now the middle child). Anyway, I don't know how to deal with it and any advise would be helpful. So thank you.