What they said is EB2-PERM not NIW

NIW doesn't need employment support. Postdoc is not a perment position so they can support your GC card application by PERM or EB1b. However, NIW is the only way you can do. Tell your boss about it. Tell him you only need a ref letter from him. In this letter, he need to tell immigration officer you are excellent and your work is benefit National interest of USA.