Sweet and salty are the survival taste sensations and the last ones to go. Yes, dementia does change the taste of food for the patient. That is why they grab the sweets and snack foods that are salty. Mom is the same way. When the care facilities eliminate the salt from their diet, it is the sweet they go for. But not all sweet is dessert. Sweet potatoes are now one of Mom's favorites. She will pick sweet potatoes and eat them. Fruit is naturally sweet and nutritious. Meat done with fruit sauces works well. Mom gobbled up a sweet lemon ginger chicken she had recently. I have even sprinkled sugar over Mom's veggies with success. Remember that some of what they do is driven by desire. They go back to what they like.... just like we do
When they pick at the food but gobble up the dessert you may want to change the diet so that the other foods mimic the dessert... usually sweet and soft.
Ensure is an excellent supplement for those that are eating less and/or losing weight unless it causes diarrhea as it did with Mom. We used it with some success to help Dad maintain his weight. It is high in calories and has the nutrients they need. It is also a consistency that is easy to swallow for those with swallowing difficulties. Letting them eat what they will and then supplementing with Ensure is standard protocol for any resident that has weight loss and/or eating difficulties.
Hope your FIL's move goes well and he gets the additional help that he needs.