Is he breaking any rules? If not leave him alone.

I guess I don't know the legality of taking water from a retention pond, but he is definitely breaking the rule of being a good neighbor.

Is it possible he cannot afford to put in proper sprinklers / pay for the water? Is there any way the neighborhood could join together and offer to pay for a new sprinkling system to be installed? I know that's a big burden on the neighbors, but it might be that he can't afford to do anything else.

Form an association and draw up rules regarding use of your shared resource. Use of pesticides? Motor craft? Noise at night? Planting of invasives? Drawing water? Docks or hardscaping at edge? You all share custodial responsibility for preserving its ecology as well as the enjoyment of your homes. Put this one action in a bigger picture becsuse you are bound to face more issues. Just because your rules may not not be legally enforceable does not mean they won’t carry weight. It’s an agreement that good neighbors are likely to want to uphold.

it's very possible that using the water from this pond is not within code either. Ask your city or county codes department o review the situation and provide guidance on how it's to be used and maintained. They may not know that this person is using the water.

There are many reasons why retention and detention ponds, mostly it's for flood management. His use of irrigation, albeit a small percentage of the total pond water, is creating an imbalance in how that pond naturally ebbs and flows with the weather and filtration processes.

I agree that an association or committee will help mitigate other issues, that will eventually come up.