That could be a gall stone or something blocking a bile duct. I had a gallstone a couple years ago in between treatments and my urine was a dark orange color and my stool almost a gray color. Luckily it passed on its own in a couple days and things returned to normal. I did see my oncologist because of it and she said it was a serious issue and needs to be checked as it can cause serious infection.
This may not be what he has but some of the symptoms sound the same. I also had back pain.
06/14-DX with FAP as 36yo Male
07/14-total colectomy, rectum removal, permanent ileostomy
08/14-DX Stage IIIC, KRAS mutant, MSS
09/14-04/15 - 12 rounds of FOLFOX
07/15-CT showed para-aortic lymph node, onc thght inflammation
10/15-DX Stage IV, CT lymph node tripled in size, 1 small lung met
11/15-FOLFIRI + Avastin
06/16-lymph node is stable, now have a 2nd lung met
01/16-lymph node is stable, lung mets grown 2mm. Still on FOLFIRI + Avastin
11/17 - no chemo since. Lung growth minimal, lymph node is stable