Gaining weight?

So this isn't for me but my dad was diagnosed with cancer last year and has since doing the radiation and chemo lost a ton of weight he is done with the treatment now but it's really struggling putting weight on and can't eat a wide range of foods because of the radiation on his throat it hurts to eat certain things or it's hard for him to swallow if anyone has any tips or advice foods he can eat really anything that may help him would be incredibly appreciated thank you.