You can always cheat by referring to him indirectly ("design chief has been knighted") or simply claim that you are addressing him informally. The Sir is formal, I do not believe you are required to use it in casual conversation?
Well, we could put this another way and say that his name has not changed, in the same way as you might address me as Mr. John Doe OR John Doe OR Mr. Doe. Mr, Mrs, Miss, are all (classically) forms of minor titles.
Anything published in the press ( news etc ) is considered formal. Not that related laws are enforced much, these days; even the BBC routinely gets it wrong.
Well, we could put this another way and say that his name has not changed, in the same way as you might address me as Mr. John Doe OR John Doe OR Mr. Doe. Mr, Mrs, Miss, are all (classically) forms of minor titles.