a job for a professional trainer to deal with. Don't risk your life-things like this get owrse if theyre not smacked in the butt straight away.
Due to this being an interesting topic, I have to say, that
I would continue you lunge my horse and push him forwards, if he starts to rear, yank his head around and in. Rearing at you, throw your whip around or something, avoid stepping back away (unless your trying to save life/limb). Stepping back is allowing them to dominateand literally show themselves as being bigger than you. I wouldn't be getting on a horse like that, until it starts respecting you. Do lots of ground work, stay next to the shoulder of your horse, not in front.
Practise throwing scary things around so your horse can get used to getting scared and let him work it out himself somehow