How must I talk to an elderly loved one about the help he needs beyond what I can give?

Talk honestly. Admit your limitations and suggest alternatives for care both in the home and in assisted living facilities. Elderly people need a lot of care and attention at times, and loosening the reins on your role doesn't mean lessening the love you have for the person. In fact, admitting that this person can be better served in other ways is a strong expression of the love you have for this person.
Be frank. Go with them to their doctor.If they refuse , call the doctor on your own and express your observations and concerns.