I disagree with the first part of your statement. The word used in that passage is "pais". The most common and thus most likely usage of that word is in reference to the younger partner in a pederastic relationship. The fact that the person using the word is exactly the kind of person who would be in a socially acceptable pederastic relationship makes it even more likely that the meaning of the word "pais" in this instance is it's most common meaning; the younger partner in a pederastic relationship.

However, I agree with everything else you said. Jesus didn't go around telling everyone their sins before (or after) healing them. He simply healed them. It would have made little sense for Jesus to choose this one instance to confront someone about their sin when he had never done that to anyone else before. Therefore, despite the fact that the boy in that passage was most likely this centurion's underaged lover, the fact that Jesus healed him does not in any way indicate his endorsement of such a relationship.