Post-Radiation recovery

I'm looking for any advice or experiences re post-radiation recovery and weight gain.

My father completed 5 weeks of chemo radiation about a month ago but is still suffering from loss of appetite and some nausea. He had a partial gastrectomy back in October and is scheduled for 6 mos of additional chemo. But the dr is hesitant to start chemo again until he puts on some weight and gains some strength back. Of course, I realize my father's nutrition is the number one priority right now but worry about delaying the start of chemo for too long.

Has anyone else had to delay chemo and/or other treatments due to nutrition? What is an OK amount of time to delay?