Well. Who doesn't like doctors! Well, seriously you can't force anyone, unless it is a psychiatric patient out of their mind, aggressive and trying to hurt themselves or others. However you can talk on his/her behalf to his doctor and see how his/her condition is serios, based on your observations, and what outcome can be expected without medical intervention, and than try again talk to your relative.
Don't force it. Discuss the reasons why the person is not wanting to have an exam. It could be fear or having had a bad experience. You could find a doctor that might be a better fit. You can not force the person.