"If it happened only once is that sexual abuse?"
Abuse is abuse, regardless of the number of times it happened. As I see the issues, it's not how many times it happened, but how did it affect the boy who was abused?
If he was traumatized, and it happened only one time, the obvious answer is yes. That's like saying, "The robber stole my money. It happened only once, so was I robbed?"