I dont think thats some one would ever be able to get the chip out..its the smallest of things and its doesnt always stay where put and can move some times ! The chip its put inbetween the shoulder blades..But then they can move.
If that is what some one has tried to do it is blimmin awful and sick
I think your best bet is to explain to the vets what you have said here and see if they can track a number..If not then get incontact with the local dog warden and say the same to them..Some one may have reported there dog as missing (That fits the look of the dog you have) to them or even the police..

As long as the wound is being treated i would hold off getting him castrated and chipped for now...

EDIT-to say may be worth having a look around some of the lost dog forums !
good luck