re: I’ve got this complete dumbass millennial neighbor who I’m trying to help

Leave him alone and let him frick his life up so he has a chance to learn. You aren't "helping" people when you shield them from consequence. You're enabling their failures to try and make yourself feel better.

If you want his grass cut a certain way offer to do it yourself and acknowledge that's why. Don't butter your ego saying you're trying to "help" anyone but yourself.