To go by the board means to fail, go to waste, or cease to exist, which is what happened to his promise when he went on a flamboyant quest for his own personal grail.
Consider the statement "He had promise once, more than I let him know, but he let it all go to waste in a flamboyant quest for his own personal grail.". Based on this "He had promise once, more than I let him know, but he let it all go by the boards in a flamboyant quest for his own personal grail." a true statement? Yes, no, or I don't know?