Suggest Remedy For An Open Ulcer Sore In The Butt Cheeks

My Dad has colon cancer that metastasized to the liver. He is undergoing chemo every two weeks to prolong his life they say, not cure it. But his tumour markers have gone from 85 to 5, can this be cured with this type of response? Also the one side effect from the chemo that is excruciating is he has open ulcer sore between his butt cheeks, he can hardly sit, walk or lay down. The Oncologist has been unable to find anything to treat this with. Is this from the chemo? Is there anything that can be done to help his suffering?