How to I stun the Egyptian Zomboss?

There is a distinct animation he uses indicating he's about to charge, taking out two rows of plants. If he takes a certain number of hits during this time, he will be stunned.

The most effective way to stun him is by using a plant food on a bonk choy at just the right moment.

As mentioned above, you have to hit him for enough damage during his charge. I particularly liked firing the coconut cannon in his face to stun him.

You might have not noticed that every time he loses on third of his health he gets stunned.