If it were me, I'd have a conversation with his parents. Are they aware of his behavior, that it wasn't just the one time, and that this can be a symptom of having been sexually assaulted. What are they doing to support him, what do they ask from the community, where is he at now, do they feel concerned, how do they encourage consent and respecting boundaries.
And then, by all means, I'd have him over and I'd keep my eye peeled like a mama eagle.
Me personally? I think I'd talk with Nick's parents first, and depending on how the conversation went, I'd make a decision to speak with other parents or not. Frankly, I'm shocked that CPS wasn't brought into this one (as it happened at school).
It's an uncomfortable topic to bring up, but it's really important (and important to be incredibly kind, supportive, and a good listener while talking with parents (stigma swirls around these situations and the stigma itself is so counter productive)).
When I was in a similar situation, it was with a bestfriend's sister and her son, so it was easy for me to talk with her - she actually brought it up to me and was looking for support. It would be a lot harder for me if I wasn't close with the parents involved, and if my children had been involved (they weren't).
I absolutely adore her son, and the interaction he had engaged in wasn't because he's a bad kid. My kids get on with him well, and he's a great playmate. I just keep an extra eye on them while they are playing, and reaffirm body boundaries, the need to stop when someone asks (stop tickling, stop chasing, stop X, Y, Z - consent gets to be practiced in play), and a lot of positive reinforcement for friendship well done.