I wouldn't use "can". Anyone can go on a diet. You might say "He could never go on a diet", suggesting that he would find it very difficult; it also suggests something he might say "I love food. I could never go on a diet!"
I wouldn't use "can". Anyone can go on a diet. You might say "He could never go on a diet", suggesting that he would find it very difficult; it also suggests something he might say "I love food. I could never go on a diet!"