Got a cold cant get rid of it

Hi everyone im new here and i have a problem my dad was recently diagnosed with esophageal cancer in July. He finished treatment and did well. The problem we have is that we are now deep into the cold and flu season and he has one humdinger of a cold, and he cant shake it. I was wondering if anyone had any ideas or suggestions to help him get over it. I wish we knew of something to help boost up his immune system. Any suggestions would be greatly appreciated. Thank you all and have a very happy holiday.