How much pet scan is useful for my son?

More info. It would depend on what disease was being looked for. Pet/ct looks at function, while MRI and ct look at the anatomy primarily.
More info needed. It depends on why your son is getting the pet scan.
Difficult to say. Pet scans are an incredible technology but not for reasons that you'd get an MRI or ct scan for. Without knowing why you feel a pet scan might be useful for your son, it's hard to answer your question. What condition does he have? What answers are you hoping to gain from the results of a pet scan?