This is a very difficult situation to be in, like walking into the middle of a story with no "right" answer. I am going to try to say this gently - even though he is your sibling, he is a separate individual who may not share your values and view of life. It sounds like this may have caused friction between you for many years.

In order to help him, you may have to set aside what you think of as the correct, normal or responsible way to live and try to view life through his eyes. To begin this, forget labels like "narcissistic" and statements like "I think that to be a member of the family..." No matter what, he is a member of your family. How you choose to deal with it is up to you.

Here are some questions to ask yourself:
If he did alternative service and you joined the military, you have had different mindsets for over 50 years. Do you have other siblings who get along better with him? Can they step in and help figure out his needs?
Do you think he may be depressed by his situation? This can be debilitating enough for people to forego normal hygiene, stay in bed, skip meds...
Has he always been a "loner", or did it start after the back surgery/pain?
Did he ever marry or have a long-term relationship? Children?

If this is new behavior on his part then maybe family intervention is warranted, but neglected teeth make it sound like an long-established pattern.
If he has indicated that he wants help to change, then intervention (family or professional) is warranted.
If he has obvious cognitive impairment, then intervention is warranted.
If his condition is causing frequent medical emergencies and family members are being called on to help, intervention is warranted.

Here is the hard part -
If he is a competent adult, seems content with his situation, and doesn't want help from you or anyone else, NO intervention is warranted.

Do you have other family members pressing you to be involved? If so YOU get to decide, based on the above, whether it is something that you need to do. You might decide to just help out the other family members, keeping in mind that you cannot change your brother.

I watched my Dad, a very compassionate man who was always willing to help, make these decisions about a couple of his brothers many years ago. He maintained close, loving relationships with most of his siblings, tried very hard to help the one who wanted to change, but had to "let go" of two who showed no inclination to change. At the same time, he encouraged us to maintain our relationships with their kids, our cousins. He taught us sometimes that is the best you can do.

Pleas share your thoughts on my very different point of view.