Texasrocks,
I just took another look at my son's letter. The LoA does say that there is a guarantee of an offer of appointment subject to... It is a two sided letter with a list of contingencies (Nomination, Medical qualification, BGO interview, etc). At the bottom of the letter it shows that it has been cc'd to the three MOCs. It doesn't actually use the term "letter of assurance" (although my son's LoA from USMA does use that term.) If that is what your son's letter looks like than congratulations are in order, he got an LoA!