My ds goes by a different name than the one we gave him. Actually he goes by a number. He is called 100. He changed it before he turned 3 and refused to answer to anything else. After a long time we just started calling him by the number and that is what he has been for over 3 years now (he is 6). He has no intention of ever going back and even when he was in school or other activities he was called his number.
We can't legally change it because here you can't be a number and he doesn't want it spelled out
He figures he will change it to a spelled out name (Hundred) later on when he is a teenager, as that is his nickname. For now he has a legal name and a name he goes by and that works out well.
His original name is his "first name" as in the first name he had. Obviously if you have a first name you need a second name. Can you tell he is ASD and takes things absolutely literally
Anyway, he has been 100 for longer than he was anyone else. He still prefers it, and it is his name so his choice of what he wants to be called. And it really suits his unique personality! At least he has a whole lot of self-confidence.