No one likes to hear this, but it's never the horses fault. They are reacting to something, whether it be bad riding, physical pain or I'll fitting tack. Since they can't speak, you have to figure out what is causing the change in his behaviour. Especially if he's never acted like this before.
Do you take lessons with a teacher on a regular basis? If the horse has changed since you have him, there is a problem either with his health or with your work, even without changing his routine, the way you ride can have provoked it, like a tiny default.
I think you should get a trainer to train not only him but mostly you two as a pair. And also that you need to solve the problems first at home and then on smaller courses so everyone is confident.
He has had a break from riding and I am brining him back into work shortly.
The only problem is that if he starts rearing again I don't know what to do. Should I get a trainer to train him? He is a 16.3hh OTTB I'm 14, when I first started leasing him he was a dream to do anything with, but he changed very quickly and I didn't change his routine or do anything bad to provoke this.