I work with dozens of children on the Autism spectrum who are like this and one of my clients is exactly the same - every time we're outside on the driveway, he will trying every car door to see if it opens, looking in car windows etc. We've managed to teach him under no circumstances is he allowed to do it to other peoples cars but the temptation is hard for him to resist but with added positive/negative reinforcement, he was doing amazing prior to lockdown. At first, we started small. Scolded him every time he went on someone else's property with the 3 strikes rule. He was really motivated to be outside so if he had 3 strikes for going on another property three times, it was straight inside which was the negative reinforcement. When this worked, we moved on to not touching strangers cars (I still let him try his family cars and my own as he does need the stimulation) - all this with positive/negative reinforcement. If he went the whole outside play with none of these behaviours, he would be allowed something classified as a treat for him. However if he did do it and passed 3 strikes, meltdown or not, he was straight inside. Usually that would be enough negative reinforcement but sometimes we will have to take away access to phone/ipad for an hour or two if that doesn't work.
Obviously I know you cant control or implement this but please try to understand that this child doesnt understand what he's doing is beyond the norm. He cant control his actions and his parents are probably at their wits end as well. I understand its frustrating but maybe you need to stop engaging with him period. Don't make eye contact, don't look at him, don't acknowledge him as awful as that sounds as then he should understand. Kids on autistic spectrum often use facial cues when looking for any emotions so if you do make contact, make sure your emotions are visible. I don't know if your neighbours son is verbal or not but don't engage in conversations, just say "you shouldn't be here, go home" or "your mum has something for you".
His parents most likely won't get offended by your comments as they're probably hyper aware of his behaviours anyway but in reality, without help, theres not much they can do. The school should really be implementing therapy. If he's doing it for sensory purposes, they could buy him toys with sensory elements, theraputy, headphones with good bass, etc.
But yeh the dark evenings have been a blessing. He spends the majority of his time riding in circles on his bike, stopping anyone who walks past and giving them the 3rd degree.
