thrombosed vein

Hi  my son has developed a lump in his right upper arm just above his elbow. He is 16.  I've been told it is a Thrombosed Vein. I just wanted to ask if any other doctor has come across this in the upper body. The doctor I took him to said he had never seen it before in the upper body and cannot explain how it got there. Does not know if any kind of injury can cause it. My son does not use drugs and has not given blood ever, so I know they can come from those things at times, but that is not the case here. Just wanting to know if any other doctor out there has seen these and should I have it removed. Can it move like any other blood clot? Or is it attached to the vein wall and won't move,etc. MRI was done.  Help,  any information would be greatly appreciated.