one possibility ... depends a lot on where he is located -- do you have the address?

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Note added at 2 hrs 32 mins (2005-09-14 19:01:00 GMT)
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Oops, wrong column ...

Your Beatitude
Your Eminence
Your Holiness

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Note added at 2 hrs 46 mins (2005-09-14 19:14:51 GMT)
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And more on "His Beatitude," for what it's worth:

A Bishop's title is "Most Illustrious and Most Reverend Lord". The words, "Your Greatness", a translation of the Latin, Amplitudo Vestra, used in chancery letters, are not customary in Italy, except when writing in Latin. On the other hand, bishops there generally receive the title of "Excellency" (Eccellenza). A decree of the Congregatio Ceremonialis, 3 June, 1893, assigns this title to patriarchs, instead of "His Beatitude", wrongly assumed by them. Traditional usage, indeed, reserves this title to the Sovereign Pontiff, one of the most ancient instances being met with in a letter from St. Jerome to Pope St. Damasus (d. 384), but in practice patriarchs still use it, and it is still given to them.
http://www.newadvent.org/cathen/01137a.htm