There are no rules when building rockets in SFS.
If his rocket works without cheats, he can build whatever he wants, how he wants.
Yes, there are techniques to make better rockets, but there is nothing written in stone like: "Nein, Nein, Nein, Nein! Boosters only 4 wide, Boosters too Thin!"
Just let him build whatever he wants, as long as it works.
And Zeeray, you don't have to be so rude with him: "I don't know If someone already said this, but your rockets are ugly". Come on...