My brother had a massive,long scar on his arm, where he was pretending to throw a javelin and his arm went through a glass door (I'm old, pre safety glass)
At some point he just started telling people it was from a shark bite and then he became the coolest person in the room.
No need to lie, but as PP have said, kids ask, mostly out of curiosity. Your response and your attitude to the scar will definitely impact his response.
My niece has a huge facial birth mark. From the beginning her parents were very cool, laid back and matter of fact about it. She is amazing, totally not bothered. She has occasionally had some nasty comments, but her self esteem around it is solid based on how her parents dealt with it.
You have the opportunity to give him a narrative to tell, I like what a PP said. I had an accident when I was a baby. I think it is cool like a PIRATE!! Then the others will take their lead from him.
By the way, that huge scar on my brothers arm, is hadrly visible now, time and growth does a lot.