Well, my dwarven barbarian that hates royals and armies would likely address him as "500 XP".
If you're not sure, the players are unlikely to be sure. To that end, I'd go with Lord General because it sounds impressive and conveys the most information.
I'll also note that when I, as a DM, do not know how to properly do something, I ask myself if that is an opportunity. Here, if the guy is somewhat of a tool, I'd make sure someone called him "Lord Easley" and then have him correct them with a sneering "That is Lord GENERAL Easly, you dolt!" Him being rude to someone over something the players and their PCs do not know would be a good way to convey a lot of information about the NPC. You could even up the stakes a bit and have the 'dolt' be new to the service and ask one of the PCs "what is the proper way to address an Earl that is a general?", giving the PCs personal involvement in the affront.