Steam Activation key

I have installed BeamNG Drive with the Activation key that I had purchased. At the time my son did not have a steam account set up so I installed the game on his PC with my steam account. Since then my son has created his own steam account. I had contacted steam support to inquire about the possibility of transferring the key to my son's account but they aren't able to transfer the key. Is it possible to have the existing key invalidated and a new key issued that I could then install on my son's steam account?

Thanks,
Stewart Downey