Be generous of your non-verbal reassurance. A gentle touch, a firm hold of his hands, or a hug may do wonders to ease the person’s feeling of fear and agitation.
See the person’s physician. In case the person is showing a severe delusion and you fear he might hurt himself, talk to his doctor. A medical evaluation may be needed to ease the problem.
Understand that a person with dementia tries to make sense of his world - the world with diminishing cognitive ability, confusion, and fear. As his caregiver, you should be able to explain to every member of the household how patience, love, and understanding can help the person with dementia.