Or HaChaim on Numbers 4:49/אור החיים על במדבר ד׳:מ״ט

1 על פי ה׳ פקד אותם, He counted them at the command of G'd, etc. The actual command to appoint the Kehatites to their respective tasks, the Gershonides to their respective tasks, etc., was given only to Moses. Aaron had no hand in these appointments. Once Moses had counted them Aaron allocated the various tasks to each of the Levites who had been counted. When the Torah writes ופקודיו, "and his appointees," this is a reference to the appointments Aaron and his sons had made. This too, however, was only in compliance with instructions given by G'd to Moses.