Just start him back out with a snaffle of your choice...I personally prefer a simple loose ring. I will use a three peice if the horse has a low palate, but most horses are just fine in the simple single joint.
IF he was trained right, the transition back downwards should NOT be an issue...however, if the previous owners used the bit to control the horse, you will have to spend time getting him to respond in a simpler bit again...the advantage of the snaffle is the direct rein pressure...I don't care how long shanks are or how harsh a mouthpeice is, curbs are very easy to evade for a horse. The snaffle will allow you to be able to pull his head around, without confusing him. I start with lateral flexion...bending side to side until a horse is soft, and bends as I pick the rein up, rather than leaning on it.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."