First, if you have not done so already, it is recommended that you consult with you father's physician about this issue.  There may be medical reasons for this decrease in appetite, and there may be treatments available.  It is also important to understand that with the progression of dementia symptoms, your dad may experience changes in abilities and interests, including with eating and food.  The process of eating may seem overwhelming and confusing.  Eating and food may not be the same experience it once was; tastes and textures may be experienced differently, affecting interest and desire. He also might be having difficulty swallowing, which is another issue to discuss with a clinician. With these considerations in mind, it is still important to encourage a healthy and nutritious diet.  Continue to offer foods that your father enjoys and finds comfort in.  You might offer certain foods in smaller pieces so that he can use his fingers rather than utensils, since using utensils might be too confusing.  Do not overwhelm his plate with too many choices; offer foods individually or on separate dishes.  Offer cues and encouragement, and sit with him while he is eating, perhaps eating yourself so as to make it a more social and engaging activity.  To make foods easier for your father to chew and swallow, moisten and soften foods with sauces or gravies.  For extra calories, consider offering snacks such as fruit smoothies using frozen or fresh fruit mixed with yogurt or juices. 

You may want to check with the nutritionist at your local hospital or in a facility to get some ideas. Lack of appetite is a problem. Sometimes it comes from illness and can get better when an infection or other ailment is taken care of. Sometimes, it means the person is "giving up" on life, as in late stage Alzheimer's. If a person's body has started the death process, then food is not desired. Therefore, medical advice is important.

Take care. We'd love to hear how he's doing as time moves on.

Carol