He is breaking the law by exposing himself. I don't know if you want to pursue legal charges, but you can call the police on him.

It is illegal to exposure yourself even in your own backyard if you know your neighbors can see you. That's what privacy fences and arbor vitae are for.

He has every right to walk on public land, but if he shows too much of himself again, tell the cops that you are him charged with indecent exposure.

https://en.wikipedia.org/wiki/Indecent_exposure_in_the_United_States