Re: Regular food
Yes, there is a preference for sweet foods in many dementia patients. Sweet foods in nature are high calorie and filled with nutrients we need to survive.
Is he refusing to eat other pureed foods that are fed to him or just not eating regular foods other than deserts by himself? Mom, if left to her own wishes, would only eat the dessert. But if fed she will eat what is put in her mouth.
The inability to get food to the mouth is one part of this disease and the inability to swallow is another. If they can't get food to their mouth by themselves but they will eat and swallow once it is fed to them then it is OK. They just need to be fed and it doesn't matter how long that takes. It is when they lose the ability to swallow the food and choke that it become a problem.
Ensure is a supplement not a substitute for food (though that is what is used in many feeding tubes). If your FIL is losing weight then his intake could be supplemented with the Ensure. He can have both at the same time. Many of the residents in Mom's facility are still on regular food (mechanical soft) but losing weight so they also receive Ensure.