I would definitely take him for a check up. Any time a new symptom comes on suddenly it needs medical attention. Too often treatable problems are allowed to continue, and often do permanent harm, because they are shrugged off as part of the dementia process.
Dizziness can be the result of Meniere's Syndrome, an inner ear autoimmune disease. Meniere's attacks are sudden and frightening; the disease can include dizziness, nausea/vomiting, tinnitus (ringing in the ears) and hearing loss and is often mis-diagnosed. All of these can be very frightening to a person with dementia who doesn't understand what is happening.
Another cause could be that one of the little "stones" that are part of our balance system in the inner ear and should be floating has become lodged in a part of the balance system and is causing dizziness. If this is the case, a physical therapist trained in balance problems can usually fix the problem in one or two sessions. Older people are particularly at high risk for this condition.
For any hearing loss issues, a good resource is the Hearing Loss Association of America, Inc. They are a not for profit, consumer based national support group for people with all kinds of hearing problems.
Michael A. Bower, ACC, Life Enrichment Consultant