Only a Whole Body PetScan could possibly detect "if there is cancer somewhere else in the body". I have never had a whole body Petscan, just head and neck.
Since you asked for opinions, I'm with your husband on "just say no" but I am very biased against Petscans because of my own terrible experiences with them. I received both false positives (the Petscan said the cancer had come back when it had not) and false negatives (when the cancer finally did come back, the Petscan said it had not). I never had this problem with MRI/CT and that is all I get now.
On the other hand, some Radiation doctors (RO as we abbreviate here)use a Petscan instead of a CT or MRI to map out the radiation field. Your husband will need some type of scan (CT, MRI or Pet) for the RO to properly set the 3D conformal radiation field used in today's modern radiation types(IMRT or IGRT or SRS) if he hasn't already had one.
Hope this helps
Charm