Lunging is a good point to cover. In general, I would just take things slowly and work him through tasks that you would expect him to be able to handle before you step in the stirrup.
My own horse gets a fairly long time off in the winter due to nasty weather, etc. He gets a "re-start" of sorts in the spring, to make sure that there aren't any kinks.
Some other maneuvers that I would put a little polish on in-hand are yielding the shoulders and hindquarters from both directions, backing up, sidepassing in each direction, as well as NH-style lunging, such as CA's Lunging for Respect stages 1 and 2. I also like my horses to tolerate me slapping the stirrup leathers up and down against the flap without flinching. An understanding of yielding laterally to the halter and bit wouldn't go amiss, either. If you have control of his feet on the ground, you'll probably have control of them under saddle.
I definitely recommend waiting to find out what's going on in your horse's mouth before delving too deeply into training. You want him to be as comfortable as possible before asking him to work, or you may have some difficulty. If his mouth is bothering him, I have no doubt that that is part of his lack of acceptance of the bit. Even working/riding him in a halter could put pressure on his jaw or sinuses and cause discomfort. Best get a handle on that before moving on.