Everyone is different, but I like honesty, being told upfront, and probably feared the unknown more than anything. He is lucky to have you on his side, supporting him, and obtaining information, The fog could be many things, and like David said, Chemo Brain, and the meds you mentioned, dehydration, insufficient or lack absorption of nutrients, and goes on. It takes time, two, three weeks for the mucus, and at 3 months I felt best, but not completely. Some side effects can last up to two years, more, but are minor inconveniences compared to cancer. Good luck.