Hope the move!
That's tricky. We had neighbors years ago who rented the house next to us for about 3 years, they had 4 boys ages 4 - 10 at the time they moved in (roughly the same ages as my kids) - - the family was a little weird, seemed nice enough but I think the mom had a little agoraphobia or something - she was friendly as long as she was in her house or yard, but much different anywhere else, and she seldom left the house without her husband.
At any rate, the boys had the worst potty mouths you could imagine, and not just 4-letter words, but really vile stuff, and making threats, but kid-type threats, not anything you'd really get upset about. The mom would frequently lock a couple of them out of the house. It was weird. A few times they made the boys sleep out in a tent in the front yard (the house has no back yard) for a night or two - even through thunder storms and stuff. Partly it was punishment, and partly I think it was just her way of giving herself some peace and quiet. But they boys always were fed well, even if they were stuck out in the tent, one of the brothers or a parent would bring them food and let them eat, they just wouldn't be allowed in the house (except maybe to pee)
I tried to talk to the mom and dad a bit, mainly just in generic terms about kids - but they seemed to think it was OK, and that the boys liked to stay outside, and that it was good for them to have a little time apart since the four boys shared 2 bedrooms. I spoke with two of their teachers twice, but the kids were OK in school, and the teachers really didn't see anything to be concerned about other than the kids using inappropriate language at times. I know they had the school social worker involved a little at one point, but I'm not sure exactly why or what prompted his involvement.
So I guess I don't have any really good advice, other than if the boy goes to the same school as your son, and you feel comfortable talking with the teachers, perhaps mention that you have some concerns about your son's friendship with the child, and see where that conversation goes.
At any rate, it seems to me that the best time to initiate a conversation with the parents would have been when they first mentioned these issues to you - - so if it comes up again, maybe ask them if they've sought any sort of professional help, or had their son tested for ADHD, or just ask them if those methods seem to be working for them, and see where the conversation goes.