Although the horse has been ridden twenty times, we don't know how he has been ridden or with what consistency. It is very likely that much of the problem is the result of being insecure with a rider on his back. Some horses that are insecure with a rider try to run away from their problems. Others are afraid to move. There is also a high likelihood that the horse is uncertain of what it is supposed to be doing.
Although this horse is eight years old, he is inexperienced. You should approach him as a preschool student. Start from the ground up. Don't be in a hurry to canter. Wait until you have a consistent walk and trot.
Think of what you are asking the horse to do. He is accustomed to moving his own unencumbered body at his own pleasure. He is now being asked to move at a rider's discretion with an unaccustomed weight on his back. At a walk, he generally has three feet on the ground to support this weight. At a trot, two diagonal feet support the weight. In two steps of a canter, he is asked to support this weight on a single foot.
This horse needs to develop strength and flexibility. The better balanced you are over his center of balance, the lower your center of gravity, and the better your body moves with his, the easier this will be for him.