R.W.
Mom of two boys on the Autistic Spectrum weighing in.
I have a 7yr old who has Aspergers (high functioning...mostly severe social anxiety, and inability to "read" social cues, very literal, and very self absorbed)
And I have an almost 5yr old who has atypical Autism (higher functioning, gets frustrated and overwhelmed over little things, is happy go lucky, blissfully ignorant of everything and everyone around him unless he is directly interacting with someone, runs away without my knowledge despite me being as vigilant as I can be with 3 kids and 1 on the way..and prefers to not wear as little clothing as allowable :-D )
Talk to the parents...chances are very high that they are completely overwhelmed and not sure how to handle the issues themselves. Work together to find a solution that is workable by both parties. Ask them what types of communication work best for him (suggest signs, Picture words, barricades etc) You could put a stop sign in your yard where he normally comes in at to let him know that it is not a good time to play. You can keep a picture of his house handy, show him the picture and then tell him to go home, you could also get a temporary fence to block off the backyard. It will take awhile to work out how to keep him from the yard when you or the parents are not out there to supervise but repetition is key...he will probably take longer to learn the "NEW" rules.
As for violence...most Autistic kids won't go after someone and intentionally be mean to them...there ARE rare circumstances when an Autistic child WILL do that but it is generally because the person they go after is in someway instigating the Autistic child. (for instance if 7yr old steals 5yr olds favorite toy i generally hear a shriek of rage followed by 7yr old yelling DON"T PINCH ME!!!) Most meltdowns are caused by frustration, anxiety, fear, hunger, thirst, etc. The problem is that unlike a neurotypical child Autistic children either can't or don't know how to tell you what is bothering them. Most meltdowns while disturbing to watch are completely about the child. he's not in a fit of frustration going to go over and start smacking someone around...he may yell, scream, throw things, fall on the ground kicking flailing etc. But generally they will pull themselves together within a few minutes.
What I suggest is that if you fear for your children's safety and he starts a meltdown (I don't use the word tantrum to describe these because he's not a bratty kid who is trying to get his way...he is a kid who has reached the threshold of what he can handle and is expressing his frustration) gather your children and keep them out of his way, call his parents and let them know that he's melting down, and ask that they come deal with him, or if you feel comfortable ask them what if anything you can do to help. Most of the time there isn't anything to be done, it just needs to run it's course.
It is helpful to my boys if I offer them a drink and a snack every couple of hours to keep their blood sugar from crashing down which can be the foundation for one heckuva meltdown. If the parents are as non-committal to their children as you think I would bet he's not getting a good breakfast or lunch in him during the day which could be causing a lot of meltdowns. I keep the 100 calorie snack bags (get them at Sams for pretty cheap...or Big lots sometimes has them for $1 a box) and water or juice handy for the boys.