He doesn't need to keep power through magical means. You said that the threat of another destructive incident is enough to keep the people subdued for a time so that is a good place to start. If he is a warlord, the sorcerer surely has followers already so the best thing to do would be installing them in positions of power in the city. If he has a stranglehold on guards, food, money and other necessary resources, he's already won. If there are any swordsmen or warriors, creating an oppressive force similar to a city watch should be enough to quash any rebellions.
After this transition phase has passed, fear really won't be necessary to keep commoners in line. Historically, as long as they're left alone, they don't care who is ruling. The main rebellious class will be the highborn who were in charge previously. If you still want to do it without magic, simply exiling from the city them will work for a time. No upper class, no upper class rebellion. Otherwise, turning a few to the sorcerer's cause, perhaps with promises of wealth or more power, will give him spies with which to keep control. It is hard to plan an uprising when any of your co-conspirators will rat you out.
A rotating set of mages could keep meditating for magic to be used when needed. If it takes six weeks to gain enough power to obliterate a building, but most people don't know this, then the intricacies of magic don't seem to be widely known in your world. In this case, three or four mages under the warlords control, by alternating who mediates and when, could keep a small city in line. Even if the warlord is not one of the mages, he would still have at least one mage with a fortnight of power stored. Depending on how much this is worth in your world, a fortnight's worth of power could be more than enough to remind people of the warlord's power. This way, you can conceal the weakness of magic's long charge time.
