Respectfully OP, you need to get a backbone. You’re wringing your hands and feeling anxious an 8 year old. You’re far too soft and have allowed this to persist for far too long already. He knows you’re a walkover.
You are the adult here, he is the child. The fact you even use the terminology that he ‘undermines’ your parenting says something. The only person allowing him to undermine you, is you.
Next time he is up there, tell him to get down, NOW and STOP interfering where he is clearly not wanted. If he persists, openly spray him with a high pressure hose until such time that the brat stops. Repeat as much as needed. Here is your chance to show your children that they don’t have to just put up with other people’s shit in life!
Also, collect whatever he has thrown, march round there, bang on the neighbours door and leave whatever it is on their doorstep when they open it. Don’t ask them, tell them to keep their child under control and away from your fence or you will continue spraying him whenever he invades your privacy.
Quite frankly, his parents are clearly shit, so you likely don’t want to be friends anyway, and he’s an arse who you probably don’t want your kids hanging around with, so no loss. No way I’d spend a penny more trying to shield him from the garden, it is time he just got telling off after telling off after water spraying until he gets the message. If he persists, so do you until such time he stops.
So go and get your life and your peace back from an 8 year old.