I also felt nauseous before I even started chemo, it's almost like preemptive nausea. I think they may call it anticipatory nausea too. I felt it was due to quite a few factors, mostly the location of my cancer and new pain medications my body wasn't used to... not to mention my nerves were shot!
So, I guess the side effects from cancer are already here, but the chemo side effects can start quickly or have a delayed onset. Nausea will be one of the feelings he may come across again, so I suggest speaking with your doctor about it-- the anti-nausea meds are pretty great!