I assume you mean stained and discoloured, rather than splintered or damaged?
Try wire wool dipped in white spirit for a start. Rub hard into the staining.
If it's really ingrained, then some simple sanding with medium paper may be needed.
You might like to finish with a couple of coats of oil....... Teak Oil is fine. Apply the oil sparingly with a rag, and rub in well. I guess you'll be re-visiting often, so re-oil every two or three months or so. It'll give you something to do while you're talking to him Mags :o)