Some sources speculate he volunteered while other speculate he was more involved with the Magitech Research than people thought.

Official sources simply say he was the first test subject but sine much of his past is unknown, there is nothing that says he was a General/Court Mage at the time of the experiment. So its possible its because of the experiment he was able to rise through the ranks.

Interesting to note, apparently in the original scenario of the game. The Magitech infusion process was always meant to be a failure that lead to insanity. Celes was suppose to have a side story about her slowly losing her grip on sanity from the process and it was through Locke's love that she was finally able to overcome it. It was dropped from the scenario for unknown reasons, most likely time constraints.