Hi Suz,

This isn't something I've experienced and this may seem a daft question but has he talked to his GP about this problem? I ask because as a cancer patient I know how cancer can come to dominate your thinking and it could be that these symptoms (especially the diarrhoea) is being caused by something else. 

I hope you get to the bottom of this and help him get his weight back to normal :-)

Good luck

Dave