Can my uncle (99) forget how to eat to the point of dying?

Hospice has some helpful material that explains the end-of-life process. Even if you are not considering hospice care, have a look at their materials.

He is in the late stages of dementia, and is not swallowing properly. So his saliva gets into the lungs and that leads to pneumonia. Food will do the same. Just make sure he is comfortable, has pain meds if he needs them. At the end, the body cannot process food; it is more comfortable not eating.