Doesn't matter who came up with it first.
If he made his own kitbash of it, its legal to do so. He only has to get premission from the author of the orgianl to release it.
As for the OP, good work!
It looks like a good way to give a kid a present his own personal starship.
For a critque though it is hard to really tell in the picture but looks like the impulse engines and pylons coulod use a different texture backing as the pattern doesn't seem to match the rest of the ship only the colour.