Hi I. Chaytor
It is entirely your choice whether you choose to say 'I cannot prepare food' or 'he cannot prepare food,' and there is no right or wrong answer.
Personally, when I fill in forms for other people I always say he or she cannot prepare food as I want to also make it clear they do not have the capacity to fill the forms themselves.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems