Hi. First is his sugar in control? When sugar is not controlled properly, it can cause swelling, which may be part of your father's sciatica.
But you really should encourage him to see a doctor about the pain he is having to find out it's cause. That really the only way to treat it, is to find out what may be causing it. It could be your father's back may simply be starting to age and causing him some pain. But you need a proper doctor to see him, run some tests and take it from there.
Good luck and keep us posted.