Once you feel that he is going smoothly when you are on the ground you should ride him again. It would be helpful to have an experienced person to help. Mount him quietly but confidently in the school where he is used to working in after giving him twenty minutes or so on the lunge first. You should not hopefully encounter any problems and by the end of a week of riding he should be working at walk, trot and canter happily on each rein. When you feel really confident dispense with the lungeing and ride him straight away.
Any hiccups go back to lungeing first.
I mean do not overfeed him and give him any excuses for being naughty and try to turn him out every day as well to keep him relaxed. Do try and find a reputable trainer to help you as you obviously need some confidence and horses are so quick to know if you are a little unsure and quickly take advantage.