PET to check status before last combo treatment?

My husband is Stage IV (distant metastasis, nothing in brain, organs, bone) and is currently undergoing Ipi/Nivo combo and tolerating it quite well all things considered.(fatigue, joint pain–particularly knees, low energy all around, diminished appetite). Oncologist wants him to have PET scan after 3rd round to see if combo is working. We asked him about waiting until after the 4th one, since it seems he’ll make it all the way through, and he said that they will know if the combo is working after the 3rd round, no need to wait till the 4th and final. A surgery/radiation treatment decision is pending the results. The distant metastasis is very large and it’s kind of hard to tell if it has shrunk. The hope was that the tumor load would be reduced before moving on to surgery. Consult with surgeon was that given the size and location, surgery is recommended either way, but results of PET would determine if the purpose is curative or palliative.

Does this seem too soon to know if the combo treatment is working? Anyone with experience on this? I’m getting very worried since the spread to the distant lymph nodes (Stage III to IV) occurred while on Opdivo alone. Not sure what’s next and want to make sure he gets full benefit from the combo so that we know. I suppose if they see further spread, would that indicate it isn’t working? Ugh, this just is so scary and unnerving–I hate the “what if’s”.

Thanks everyone for any insights!