With lunging...... I'd start in a roundpen and just keep him away from you and moving, praising him a lot. With the lungeline.. I'd walk with him on the line and slowly ask him to move away from you. Some horses are terrified of the whip and when they see it they think "omg there's the scary monster that chases me and hisses and tries to snap my tail off!" so sack him out with the whip. Look up some videos on youtube about how to sack out properly.. the key is taking your time and giving the horse space to figure it out for himself.
BUT PLEASE, if he is rearing with a line... oh my goodness girl, get him walking on the line perfectly before you ask for anything more! Drive him forward... don't make the line too saggy or you'll end up in trouble. Please be careful!!!!!!!
Make a game when you ride. Ride around the span of the arena first, then do one large circle and go the other way. Do little soft half halts in between to get your horse on you when you change directions. Then go again around the arena twice, now do two circles. One at one end, one on the other. Remember those halt halts... now walk him, and put him into serpentines, half halting when you change directions. Half halt again and ask for a soft trot. RIDE THE TRANSITION PLEASE :) then do a gentle figure eight, remembering those transitions. Then half halt and wind down into a soft but large walk. then make him sidepass across the diagonal of your arena and get him bending on the rail. Then do a large half circle and ho him. Spend a minute, rub on him. Then ask him to back up 2 steps, walk forward four, then back one, and walking on forward. let him have a loose rein and do three large loops across the span of your arena, asking him to bend his neck and drive him forward with your seat until you are satisfied with the walk, gather up your reins and ask for a trot again. Trot a large circle taking up 3/4 of the arena, and work down to a smaller and smaller circle.. ONE HE CAN HANDLE without falling apart.. then get him back on the rail and go again across the diagonal.
Notice what I'm telling you to try? A variety of things... circles with bend, changing directions, asking for quick lead changes.. all of these things, as long as you are driving with your seat, supporting with your legs, and asking for a bend, will get him building up muscle and will also get your creative juices flowing too. Just remember those half halts, and praise him when he does what you ask :) Don't let him have any breaks until YOU feel your legs becoming bouncey and weak and you feel his neck start to tense up a bit and his feet getting slopy. That would probably be enough.
It's going to be a lot of hard fun work :)
But definitely get ahold of some side reins and lunge him gently (at first) in them.