I haven't had chemo myself (yet), but I know my friend lost a lot of weight on it, so I think going in with 10 or 20 lb to spare can't hurt.

Sugar consumption has no demonstrated effect on prostate cancer, but reducing sugar intake can be helpful for avoiding secondary complications like diabetes. During chemo, however, he should probably eat an unrestricted diet of whatever he feels like (if he feels like eating at all) — confirm with his medical oncologist, of course.