Your mod? What, exactly, is your mod about?
Regardless, there's not much that needs to be done. Unless he actually has a hand in getting the mod put together, he can't do anything to prevent you from completing it, and I doubt anyone will hold a grudge over you not including one of his ideas. It's your mod, after all, and you not only don't need to include advice from everyone but you also can't include advice from everyone. If you really feel that he deserves an explanation, just tell him that adding an extra unit takes work, and that if you added all the flavorful-but-meaningless units you wanted, you'd be completely innundated in work, and the game itself would have too many pointless units to be playable.