I believe this type of unusual situation requires a really experienced low carb / Keto practitioner like Dr Trudi Deakin or Dr Eric Westman.
Of the 3 macros to potentially add more weight the carb route has the side effect of increasing fasting, resting and post prandial blood glucose and raising insulin, when your husband is most likely already insulin resistance. Maybe due to the cancer your husbands appetite is suppressed (I know my Mother's was), so he might be in an accidental caloric deficit.
I would try butcher sourced minced lamb, made into burgers, a tasty way to get fat and some protein in good quantities, Firing off an email to Trudi might be a good first move
admin@xperthealth.org.uk https://www.xperthealth.org.uk/Home/Our-Team).