The general advice is to try new cuisines, and to add more herbs and spices. For my aging father, I am looking for condiments and foodstuffs with a strong, distinctive taste that he could add to sandwiches and meals, in a easy way.
Miso paste. A Japanese staple that gets its strong taste from a bacterial culture. Can be used as a soup base, or to be spread on a sandwich in combination with another savoury topping. https://en.wikipedia.org/wiki/Miso
If he is better at drinking than eating you can sneak a lot of nutrition in to shakes, sweet or savoury. They sell tins of powder especially for this purpose but a plain protein powder isn’t a bad base to add to.
If he is really having trouble with food a little cannabis can help a lot if it is legal in your jurisdiction. I recently joked with a friend with late stage cancer who was hesitant to eat a brownie that it was why family sized pizzas and home delivery were invented. She declared it a miracle later.
For sandwiches etc there are any number of pickles, relishes and chutneys depending on his tastes, many go great with cheese. Roasted garlic makes mashed potato interesting and caramelised onion goes with a lot of meat, I add a bit of chilli for zing.