My son has a bone sticking out on his arm?

my son is 2yrs old and he was taken to hospital a year ago with a bad arm kept cryin when enybody touched it ect...so we took him to the childrens hospital and they kept him in for 4days because i could not explane how he had done it they did x-rays of his arm and wrist they sed he had a small break so they put it in plaster to see how he went they eventualy let us home,we went bk to the hospital about 4weeks later and they did a 2nd x-ray then they said ther was nothink wrong at all took his plaster off and sent us home! now a year on ive notised a small lump on his wrist its hard and it dnt hurt him at all..ive spoke to the doctor and they said take him to A&E i dont kno what to do???