Idea theft? Sue 'em in Britain.
Not in Britain, go to Britain, see above step.
No chance of ever going to Britain, execute one-liner, /sunglasses.
On topic: the fact that he's a robloxian doesn't mean anything, people only think it means something because idiots and trolls think it does and start pointless kerfuffle. Frankly we're suppose to be on good terms with them since on both the Blockland and Roblox forums it's a bannable offense for someone to start an inter-forum flame war.
Now if he really did steal your build you should use that topic of your build and testimonies from key-witnesses who frequented your server as evidence.