Capping a broadcasting career that spans five decades, William M. Pepin will step down as general manager of WWLP-TV, 22News, on July 30. Pepin, who celebrates his 67th birthday on Aug. 11, recalled that his fascination with television began as a 5-year-old watching a Channel 22 crew cover the Holyoke St. Patrick’s Parade. “I was mesmerized. I watched them instead of the parade,” Pepin said. “The next year, I did the same thing again.” As a youth, he hung around the old WREB-AM, 930, radio offices in Holyoke before they hired him to work on the board at the age of 16.