My Dad's dementia deteriorated significantly after placing him in AL. Is this common?

Jennifer, yes a sudden decline when moving someone with dementia occurs more often than not, not at all unusual. Moving is very difficult and stressful for most people. Imagine how much those feelings would intensify if you had no idea of where you were or who all these other people are. It is not your fault, most of us reach a point where keeping them at home is no longer possible for any variety of reasons.

Agitation is often a result of these moves and requires close monitoring of behaviors. It is not at all unusual for the facility to require a one on one caregiver be hired by the family. Adjusting to a new living situation is nearly impossible for some.

Jennifer, each change is huge for someone that has dementia. I'm not sure what your time frame is, but they frequently settle down and recover some of the previous status. Continue to visit and be supportive so he knows he is loved and hasn't been abandoned. Talk with him and point out certain 'landmarks' of the AL. It will be all new each visit but he may get excited about things each time.

One family of a resident in my Mom's AL brought in a bird feeder and hung it from a tree. Each visit, they would fill it and then bring their Dad's chair near a window where it could be seen (or bring him outside when weather permitted). Try to make him comfortable and with any luck, things will settle down.