My Dad (90) has started coming out without his clothes on. Is this common?

This is a common problem, and there are specially made 'jump suits' to mitigate this from happening. It's easy to help him put it on, and prevents him from stripping so easily. They're also very comfortable. I would check out AdaptiveClothes website

There are some who will strip, not sure if that is what you mean. Dad did and the nursing home told us to get him one piece coveralls. This was a long time ago and it was a pain changing his disposable underwear. Now they have specially made clothing for strippers. Look at Buck and Buck and several other companies that make adaptive clothing. The snaps and zippers are inaccessible or difficult to open. Dad had all sorts of hallucinations and dreams. I hope your father doesn't go through that.

Not common. Dementia is advanced. I hope he is not living alone.

My father has dementia and when we were growing up he was very private about that stuff...he now walks around in his underwear in the day or depends at night time...so not his true character.

Need some more info
..Any recent surgery or hospitalization? Is this at home? Is this the First time this has happened? Has he been diagnosed?