How to encourage father to continue chemotherapy

My dad just finished his first round of chemotherapy to treat a retroperitoneal sarcoma. The chances that chemotherapy will reduce his disease are about 30-40%. Although it first seemed as if he was tolerating the chemo relatively well, several days ago he developed very painful mouth sores. Now he is unsure about whether he wants to continue with another round of chemo, because of the discomfort he his having. I completely respect his ultimate decision, but of course want him so much to do whatever is possible to increase his time with us (my mom, brother, and both my brother and my families, including 5 grandkids). I would love to get any advice, or words that I could use to help encourage him to keep going with the treatment. Thanks so much for your help.