"The children of my husband's nieces and nephews are what degree of cousin to my son? First cousin once removed?" Janie
Let's see if I can figure out it out.
Your husband's nieces and nephews by usage are also your nieces and nephews.
Your son is also your husband's son, by usage. Therefore, by usage, his nieces and nephews and your son are first cousins because they are of the same generation.
I don't think 'removed' applies, since there is no blood relation.