Your doctor shouldn't be offended -- if it is really true that he is a "close family friend." Your doctor should be a professional, who is not so prickly that your getting a second opinion "hurts his feelings," or something.
You might also think about this: If you are so afraid of "offending" your doctor by getting a second opinion, should continue to consult, at all, with this physician? Your doctor should and should have been the one encouraging you to get a second opinion in the first place!
Despite the Hippocratic Oath they take, which comes from ancient Greece, doctors are not appointed of Apollo or Athena.
Good Luck with everything.