It's Son. It isn't a formality like they were considering below, that's his actual "last" name.

Undisclosed I think, don't think anybody has a second name in Dragonball.

I think thats just a formality theres times when hes called Son and Son-Kun in the manga.

"Goku, known as Son Goku (孫 悟空?) in the original Japanese-language version,] is a fictional character, a superhero and the mane protagonist of the Dragon Ball manga series written by Akira Toriyama"

Yeah, I aint denying thats part of his name, im saying it's a formality, like "Sir" or "madam", his names Son Goku, but it ainnt his second name.

In Japanese I believe you put the "last" name first.
such as
Son Goku/Gohan/Goten

You do, but as much as it's like this I'm beyond sure that in English his name isn't Goku Son, I think it's a formality, I could be wrong though.

we could both be wrong, and if that's the case.
Then I'll see you in the sinking ship

You too man, till we meet again :)