LO pushing himself out the bath seat help!

Tonight my 4 month little boy figured out if he gets his feet in the right place he can use his impressive core strength to push himself out of the bath seat, consequently giving my poor husband a heart attack when he had to catch him before he slipped into the water. He's so active and tall, he's trying so hard to sit up unassisted and sometimes he's even able to push up onto his legs trying to stand no matter how tightly I hold onto him. Obviously this is a massive safety concern when it comes to the bath. Does anyone have any recommendations on what to do or any bath seats that might keep him more contained until he's able to sit up properly as I'm too nervous to put him in the bath without anything just yet. We have an angelcare newborn one at the moment