Standing in tub

What do you do when your child starts standing in the tub? I'm so scared he's going to get seriously hurt. I keep sitting him down but he will tantrum and throw himself backwards which could obviously hurt him even worse. The other day I just barely caught him because he threw himself backwards in there. I watch him like a hawk in there but he's so fast. Any tips?

Also any tips for rinsing his hair without a tantrum? He throws a fit and ends up with water in his eyes 😔 he makes it so hard.