1. Don't hold players to their alignment. The alignment could be "wrong" and nothing bad will happen. Just ignore it. Treat it like a spelling error. Like when you glance at someone's character sheet and it turns out they've been playing a "rouge" for the past four sessions.
2. Look at the special features of the various backgrounds. Make him pick one. If he's a noble, then sure, he can hobnob with the people in the palace; and if he's a folk hero, he can lean on the people in the village for various things; but not both!
The background features were designed to help guide this sort of non-combat story interaction.
3. When he says what he wants the character to achieve, be very clear and explicit about what it will take. Like if he wants followers so loyal that they will lay down their lives for him, that's going to take some incredible inspiration and/or brainwashing, plus a large pool of potential recruits to draw from.
"Other Rewards" in the DMG (p.227) is a good starting point: Maybe for completing quests he gains some sort of widespread recognition and, eventually, followers. Talk to the other players, too, about what kind of stuff they want for their PCs. Maybe as this guy is building his army, other PCs are building airships, researching custom spells, or raising a family?
The loyalty system in the DMG (p.93) is a really good mechanic for communicating this kind of stuff to the player. Straight-up tell the player what the NPC's current loyalty is. Let them roll the dice for increasing/decreasing loyalty. I'd adjust the system by tying the follower's max loyalty to this particular PC due to personal loyalty. And I'd allow the max loyalty to increase or decrease a little based on the character's reputation. This ties in to the "Other Rewards" suggestion; you can say things like, "If you solve this quest for the villagers your followers' max loyalty will increase by 1." Or, "If you go back on your word now, your reputation will be tarnish, and your followers max loyalty will decrease by 1." This is kind of similar to increasing and decreasing Honor (DMG p.265) except it's a more general reputation than just honor.
The point of putting numbers on this stuff is to communicate clearly with your player. Numbers will show him very clearly that he WILL get what he wants, it just might take a little while. They also help you, the DM, to plan this stuff in advance. Like, you could declare that max loyalty can only get at most +1/-1 per session, or that each follower can only get loyalty increased once per week, or something like that. This will let you pace the game how you want. If you set up a clear game mechanic in advance like this, it will help avoid disagreements about how NPCs "should" act.