This is a fairly detailed document on hypersexuality in dementia produced by the Royal College of Psychiatrists:-
http://apt.rcpsych.org/cgi/reprint/11/6/424.pdf
"Sexually disinhibited behaviours are quite common (prevalence 2–17%) in people with dementia,
occurring with about equal frequency in men and women. Assessment of the behaviours, the contexts
in which they arise and their risks is essential. It is important to manage the environment and to
educate and discuss the issues with carers and families. Behavioural measures are helpful, although
no specific behavioural intervention has been shown to be effective in this area. Several classes of
drug may help to control the behaviours, but all are potentially harmful and none is licensed for
hypersexuality in this population."
"Neuroleptics (Antipsychotics)
Neuroleptics are probably widely used in managing
sexually disinhibited behaviour in dementia,
but
there is very little published evidence of their efficacy
or safety in this group.
Recent advice in the UK has
emphasised the increased risk of cerebrovascular
events in patients with dementia on olanzapine or
risperidone. Benperidol has been used in the
treatment of paraphilias (Field, 1973; British Medical
Association & Royal Pharmaceutical Society of Great
Britain, 2005)."
Reading up further on Benperidol it looks like a very strong antipsychotic drug. If he does really need to take medication for this issue, I'd try and get your father on something milder. Obviously I'm not a doctor though and I'd say a psychiatrist is the best person to advise.
As the document states though, it's important to find out the cause of the behaviour first and the extent to which it is affecting others.