My son fell from a bunk bed 2 weeks ago. He ended up with a large bump on the back of his head. The Dr said he was fine, but I'm worried since its bee?

Not uncommon. It sounds like the kid accumulated the equivalent of a bruise under a portion of his scalp (just above the skull bone). When this happens, there is no way for the blood to be absorbed easily (no nearby blood vessels) so it can remain and even form a calcium filled lump. It does not mean something hurt the brain under the skull. If the kid acts fine he likely is.It may take months to fade.