My personal opinion:
You are going the wrong way.If this is what he says, then either he is not good or he has serious doubts on your chances for NIW.Your lawyer wants to play it safe depending on the strength of the letters.
A capable lawyer who understands very well the process and nuances of NIW, will not ask you to furnish the letters first and THEN decide whether he/she would take up the case or not.He/she should assess your chances on the basis of your CV,publications,awards,etc and if good enough for a successful NIW, he/she would take up the case (and ofcourse fee),give you sample reco letters and thereafter, you need to contact the writers and get those letters.
May be it is time for you to contact other good attorneys to get their honest opinion on your chances first and then proceed further.