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Hello. Thanks for the question.
The pool water is not too bad for him. The chlorine levels can lead to some GI upset (vomiting, diarrhea). Other than that, the water is pretty diluted where it's not going to be toxic.
I would try to discourage this as much as you can. I know it's not easy when they are outside and unsupervised. You may want to have a bowl of fresh water outside for him so that he is less tempted to drink the pool water.
I hope this helps, let me know if you have any other questions.