When your parent's in pain, it's important to pay close attention to when the pain occurs and what seems to make it worse and better, then talk to the doctor
and describe it as best you can. If the pain is worse the week after chemo, it sounds like the chemotherapy drugs may be causing it or making it worse. The fast-growing cells that line the intestines and stomach are among those killed off by chemotherapy drugs, which is why so many cancer patients experience nausea, vomiting, and stomach pain.
Is your father being given an anti-emetic drug such as Emend or Zofran before and during chemotherapy? They work very effectively to reduce nausea, vomiting, and cramping. However, some drugs are not considered safe with all types of cancer and all chemotherapy regimens. Your medical team should include a chemo nurse who can answer specific types of questions about your father's chemo regimen, which anti-nausea drugs have been prescribed, and when -- and how strongly -- he can expect to feel nauseous.
Many oncologists now recommend integretive approach to cancer treatment, which can include some nutritional supplements to support digestive health. Have your father try taking probiotic supplements, which help restore the balance of natural flora in the gut. Folic acid and glutamine also promote digestive health. Make sure he gets plenty of protein (you can add whey protein to smoothies and drinks) and omega-3 fatty acids from fish or flax seed.
Most cancer patients find that nausea and stomach problems continue on and off throughout chemotherapy treatment. More information about nausea and chemo is available here.