While working under Father Thomspon, he learned of his techniques, and was with him during one of his failed attempts. They were also joined by Men of Letters initiate Josie Sands. In this particular incident, Thompson got close to figuring it out, but failed and all three watched the attempt destroy the demon and the host.
After Father Thompson was murdred by the demon Abaddon, Father Simon inherited all his findings and kept hold of them. He became a priest on his own, and was still alive in 2013, when he was visited by Sam and Dean, who were investigating Father Thompson's experiments. He gladly told the hunters all he knew and handed over all of Father Thompson's findings and records.
The actor who played Father Simon, Donnelly Rhodes also played the character of Shaw, the lone survivor of a Wendigo attack, in the episode Wendigo.