I did jury service for someone being prosecuted for waving his todger around in similar circumstances, though with slightly less gardening involved.
Tell him you can see him (it) and if he doesn’t stop, tell the Police.
IANAL, but I don’t think that would work. I’m pretty sure it has to be proven that he is naked with the intention of causing offence, or something, which would be very hard to prove indeed, in his own house. Unless he was, y’know, doing a bit more than just walking around.
So, stop looking seems a good answer. Although a note probably wouldn’t go amiss, he’d probably not be too happy that he’s strutting his stuff unintentionally… Personally, I’d generally be a little averse to deliberately aiming lasers at other men’s penises, but whatever floats your boat I suppose…