Severe burning

My husband was diagnosed with colorectal cancer.  He was border line between late stage two and early stage 3, so they treated it as stage 3.  He had a chemo, radiation, surgery and a temporary colostomy bag, then surgery to reverse it, and was hospitalized 3 times between the two surgeries for blockages due to scar tissue.  He is in remission, treatments are over, however he is still dealing with residual affects.  I can't seem to find anything online so far.  

The issue is extreme pain when using the bathroom.  He won't go for anywhere from a day to a day and a half, then in the middle of the night (for the most part) he will be up, going and going and in severe pain.  He says it burns and feels like acid is running thru him.  We are being told its from the radiation, and the dr has offered advice to use something topical, but it seems to be burning from the inside and out.  He has also described the feeling as if someone has taken razor blades and just cut him all over down there, and a kind of mucus that burns beyond belief.  One nurse said she thinks it is coming from a medication he is on, Metformin, but he has had this issue before being put on that medication (for sugar).  He is a strong man, he is a 5th degree black belt, but I've seen this pain bring him to his knees.  We were so happy and lucky to have had great doctors, and happy there is no more sign of the cancer, but it is never really is over, is it? Learning the hard way.  Our happiness was short lived, after seeing him go thru this with no clear answers.  His scans don't show anything.  And its rare to search for something on the internet and find -0- about it. 

Does anyone know what I am talking about? What this is? What can help? 

Thank You

Laurie