You have told his parents what the child said.
So it is now up to them to try to find out, if he had just heard oral sex, has seen a porn magazine or video or if someone has actually attempted or done what the child said.
I know this is very difficult - children do sometimes put words together and arrive at a sentence that shocks the grown-up who hears it WITHOUT the child having any idea of what the sentence they had overheard somewhere means.
On the other hand we all know sexual abuse of children occurs, so you certainly did the right thing talking to his parents and I do understand your anxiety.
On the harmless spectrum is the young couple I knew when we were all in our twenties, who had twin sons aged 3 and were contacted one day by a very concerned nursery school teacher. She had noticed the two boys sitting on each their own potty, panting with their tongues out, and, as she decently expressed it in obvious embarrassment "playing with themselves",
The children's parents truthfully explained the matter thus: in the boys' bedroom there was a picture of a puppy with his tongue out on the wall at the children's eye-level when they had been placed on their potties and told to stay there until they had" done something". The children were accustomed to panting like " the puppy on the wall" while waiting for their parents to verified that the potties had been used.
I trust there is as innocent an explanation in this case too, but you do need to leave it to the parents to deal with, though you are justified in asking whether they have found out where in the world these distressing sentences had come from.I hope the child was playing with words and landed on an unfortunate combination, but if anyone has been licking him anywhere at all, even if only the dog, a stop must be put to it.