I"m of two minds on this...one is it's a shame that the neighborhood kids do have to see this. And it's too bad he's such an immature grump that if someone asked him to mow this is his response.
The other is, it's his private property and if he wants to let grass get >10 inches, well that's a shame too - but it's his property.
Well I sure wouldn't want to live next door or across the street from the kook.
IMHO, he likely has deeper emotional problems than just being an "immature grump".
Time to start keeping a closer eye on the idiot, just in case he needs to be carted off to a padded cell.