Stomach Pain and Folfox/Keytruda?

My husband has 2 more Folfox/ Keytruda treatments to go. His most recent CT scan on Sept. 12, showed no growth, no decrease in the tumor. No mets. He is stage 3, HER2 +. Not a surgery candidate, and 65 years old. He was diagnosed in Dec. 2022 as stage 4, and had chemo and radiation Feb. 2023, which shrunk the tumor. He went from not being able to eat because of pain and obstruction, to mostly being able eat if he takes Ativan and Zolfran right before eating. He also takes Prilosec. He has Oxycodone 5 mg for pain, which hasn't been helping the last few days.
Has anyone had stomach pain as a side effect from Keytruda? If his tumor hasn't grown, why would he have an increase in stomach pain after this round of Folfox? He's felt good until this last infusion. He was supposed to get fluids before treatment, but they forgot. The side effects were amplified this time. Could his stomach pain be caused from not getting the fluids before treatment?
The Oncologist said he is pleased with the results, so I'm hanging on to that, but I'm not even sure what to ask the Oncologist. Does he need stronger pain killers?