If you haven't already done so, you might want to start bringing him his favorite music from a key time in his past – perhaps some Frank Sinatra ballads from when he was courting your mother? (Just guessing here – my parents used to go all ga-ga whenever they heard Ezio Pinza singing Some Enchanted Evening... anything from South Pacific made them happy!) Sometimes powerful associations with love and joy will get through the fog.
Soothing aromas might do it too. The nose sometimes knows what the neo-cortex has forgotten. So perhaps you could supply him and his staff with some soothing essential oils that can be sprayed on his pillow or applied to his skin before bedtime, or once he wakes up.
Therapeutic massage might speak to his muscle memory in a way that words no longer can. Gentle, nourishing touch or energy work bypasses the thinking brain and literally gets through the skin.
And the soothing words and calming music on our guided imagery for Healthful Sleep could help with his sleep too – again, the voice tone and music get through to the primitive brain even when the words have lost their meaning. Same with imagery for Stress during the day.
In other words, appeal to his senses. You may be able to connect with him there, and soothe his anxiety in that way. It's worth a shot.
And given what you're dealing with, you might want to consider listening yourself to either my guided imagery for Caregiver Stress or Lynn Joseph's excellent set of audios called Emotional Renewal Guided Imagery for Caregivers: Looking After Yourself While Helping a Loved One.
All best wishes,