Knot on back inside of knee. ?

My son has an odd knocht on the inside backside of the knee. (not the actual back..but on the back corner) It feels like a bone sticking down and you can actually push your finger up under it. Also, every so often his knee will pop loudly or lock up (then pop). DR said they have never seen anything like it and is in the process of finding a specialist. (I am a little upset that they didn't even x-ray it). Anyone know of what this may be? It is very hard like a bone. But I am not sure.