How did he get into your pool? Shouldn't it be fenced? But he should be supervised by his parents aswell.

There’s so many things wrong with this I don’t know where you would start. You might need to make a call to docs because I’m sure you don’t want to come home to a child face down in your pool. For the time being make sure the fencing is adequate and lock the gates with the key. If the parents are approachable tell them you are worried for his safety and he shouldn’t be running around your yard and especially not near the pool.

Get a fence put up. Report the neighbour to child protection. Its a health risk for all involved especially an undressed child. That is not appropriate and child neglect to not have the child adequately dressed. No supervision being in someone elses yard again. Lack of parenting.