Or you could claim your son has a landscaping business, buy the tractor tax free, and your son could mow the neighbors yard just to keep up the illusion. Who's to know? What agent from what branch of government will come knocking on your door, checking? Does the business need to be sucessful to qualify?
It isn't any kind of "claim" that allows one to buy tax free, it's a tax free cert from the business who registered for it. So he would not be able to buy the tractor tax free unless the son's business actually registered for and received a tax free cert.