Here is a tip to prove that this was your build:
Hide in at least two locations in the inner workings of the build:
<insert build owner's name here>'s build
Or something along the lines of that.
Or even better:
-Pick a random brick
-Memorize its location
-Make an event that displays on the screen:
<insert build owner's name here>'s build
That way, if they try to put it in gallery saying its theirs; you march into that topic with a screenshot (a video is even better!) of you showing this in the build.
But until he claims that he made it, its not stealing. Be an adult, join the server and request that he displays credit for your hard work; Instead of posting it on the forums.