"Sir", not "Mr." if you're going to be pedantic about titles ;)
replyEdit: Oh and he has multiple honorary doctorates (at least 6!), so would be just as much "Dr." too!
It is not usual to call people with an honorary doctorate "Doctor" except in the context of the awarding institution. Most likely the awarding institutions will have actually specified that the recipient should not give anybody the false impression and I can't imagine Tony is the type to do otherwise.
replyHis title at Oxford was 'Professor', and he was addressed as 'Tony'.
replyHe made incoming DPhil (PhD) students a cup of tea individually in his office at the Computing Laboratory. It was a small group, but still I appreciated this personal touch.
Lol you are totally right! ;-)
replyI am normally a casual guy but for a giant being a bit more formal (pun intended) seems appropriate. Or maybe I am a nerd through and through :-)