As a mother to a daughter with Autism, I don't believe that this boy's behaviors were out of control or horrible. His actions were for the most part age appropriate and typical in this situation if not a little impulsive but he was playing with your children and enjoying the play time. And I have to say that I think perhaps you over-reacted. Maybe the dad wasn't as sensitive to how irritated you were getting, but that's not the boy's fault.
Kids with Autism are very much aware of their own personal space and personal boundaries, but it's nearly impossible for them to "put themselves in someone else's shoes" (hell, they don't even understand phrases like that... metaphors and hyperbole and similes) and that other people need personal space. They just don't until they're older, and then they have a hard time judging what that boundary is and when it's appropriate under which circumstances.
They also have a tendency to really connect with children that are younger than they are because they're more similar to younger children, even babies sometimes. I often think that my 8 year old daughter is stuck in an extended toddlerhood with many of her behaviors and emotions.
There are these... social impairments, if you will, that are very difficult for people with Autism. If this boy is high functioning, then he probably has Asperger's Disorder and therefore social interaction would be extremely difficult for him and he would need a lot of guidance, correction, and encouragement. Something you might not have realized at the time is that for that boy to be the one to initiate play was a huge, huge deal and that THAT is why the father didn't discourage him. The boy didn't do it to make you or your children uncomfortable, but because he was playing and having fun WITH your kids, and he actually initiated play. I wish my daughter could do that. She has social integration therapies at school and is working very hard toward being able to do what this boy did, even if he wasn't quite within etiquette.
Your kids were never once unsafe.
EDITED TO ADD: I'm going to add my private message response to you, since you were so kind as to ask for more help from me. I also want to make it clear that I never once suggested "giving the boy a pass." I have two neuro-typical daughters in addition to my daughter with Autism, so I understand this question from all perspectives, trust me. I NEVER just give my daughter a pass because of her Autism... I'm constantly teaching her boundaries and how to best behave socially and it's a work in progress every day.
Now here's what I responded with privately. All of my responses to this issues have been solely with the intent of offering some insight, not to suggest giving a pass. Just trying to help ease the anxiety and discomfort because so many people simply don't know enough about Autism and the people who have it.
I honestly believe the "vibe" is that he's different due to the autism. I know what you mean, actually, because while my daughter has Autism it's rare for girls to have it. And when I meet little boys that have Autism no matter where they are on the spectrum, it's a different feeling. My BFF has a friend whose son has Autism and is mild, but there's an uncomfortable feeling that's not there when I meet girls that have it.
It really is true that children with Autism have very little impulse control. They also have no natural awareness of social boundaries, and if something is fun and getting a silly reaction from someone else to boot then whatever they've learned about social boundaries is going to fly out the window for a while. They may know the rules apart from situations, but when in the midst of one they don't have that automatic recall and ability to think, "Oh wait, she's telling me to stop, I'd better pull back." or "She's getting upset, I'm going to get in trouble if I don't stop."
And one reason for that is that he literally couldn't read the tone in your voice or your facial expressions or body language. It wasn't him being obstinate or trying to be inappropriate with your children. He wasn't trying to ignore you. He just... couldn't read your urgency because it wasn't exaggerated.
If there's a next time, and he doesn't obey your requests, then you need to address his father and make it clear to his dad that you really need him to stop for ____ reasons. "I'm feeding the baby and she'll get a belly full of gas/get off of her routine if I can't finish feeding her without distractions." He should understand about routines since many, many people with Autism thrive with routine and don't do well without one. If he touches your daughter in an appropriate way and he doesn't stop, then tell the father. "Joe, I've asked Sam not to touch Ella around her private areas while they play. It's really important to me that you reinforce that with him so that they can continue to play together, or I'll start to get very uncomfortable."
I'm usually right on top of my daughter and very socially aware of what she's doing, trying to guide her and teach her... it's not easy. These things don't come naturally at all to these kids. I do think that the discomfort is more about how we as adults have been taught to think about and interpret when someone touches us inappropriately. It's very hard to turn that "off" when coming across someone who is doing something inappropriate but for very, very different reasons. Reasons that don't have a horrible, evil ulterior motive.
Does any of that make sense?