Rosie. Your horse has moved from his old place, where let us be honest, you probably don't know what happened to him to your place - where he knows nothing and noone. Sellers of horses aren't known for telling the whole truth. He has not yet made a connection with his new surroundings (although interestingly he did run back to his stable). WHen you take him out, he knows you are a different rider and that you are going to take him where he has not been before. So he gets frightened. When he's frightened he will think of running away. He will also be unpredictable.
At this stage be very carefull he doesn't hurt you even if only accidentally.
WHere do you start? - at the very beginning. At this early stage you have to give him the benefit of the doubt.
Your first job is to ease his fears. Try to establish a new routine with him and keep to it. Groom him - often. Make sure it is you that gives him any feed - and cut down on the energy intake. Restrict his access to fresh green grass. Watch his waistline - use a measuring tape. Stroke him, talk to him. Wear the same clothes every time. He must learn to recognise your voice, smell, body language and touch. He must come to accept a new routine.
Then start a programme of work in hand. Walk him round in a confined arena. Stand at his head so he can see your face. Use the pressure on the lead rope carefully - try to use as little back pressure as possible.
Walk him past scary things - bags, bags, signs. (When doing this work, wear a riding hat and good tough boots and don't wrap the rope around your hands)
Repeat the same routine every time.
Try not to damage his mouth - use a training halter which works on the nose and poll.
Then swop to lunging him. Just walk & halt. Then trot. Not too long though. Leave out the canter for the time being.
The principle I am suggesting is that he has got to lose his fears and put his trust in you. But you have got to earn that trust. No loud voice. No waving arms No crop. But you must be very positive if he does anything to hurt you - but don't get angry. Chastise him if necessary but by tone of voice - nothing physical.
These first few months are very important with a skittish horse. The work you put in now may save a lot of pain and heartache later on. If you can build a bond with this chap, then you will have won. Once he starts to neigh when he hears your car, then you are beginning to win.
PS I cheat. I always keep a carrot or a small apple in my pocket. As I say hello I give my horse ( a skittish mare) something to nibble. As I leave I always say good night and again I give her a small horse biscuit. I get told off by every body but it works for me and my horse. She's got me round her finger now - but it has taken a year.