Weddell began his coaching career as junior varsity and varsity assistant at Angleton High School from 1973 to 1977. He then went on to coach at Victoria High School in Victoria, Texas for twelve years, first as an assistant, since 1982 as head coach. Weddell guided the Stingarees to a 47–32–1 record, winning district championships in 1986 and in 1989. He was twice named 26-5A Coach of the Year and won 35 of his last 40 regular season games. In 1990, he started coaching at La Marque High School, where he turned the Cougar program into one of the perennial powerhouses in Texas high school football. From 1990 until 1997, Weddell coached the Cougars to three straight state championships and five consecutive appearances in the state title game. La Marque lost both the 1993 and the 1994 title games against Stephenville High School, which was then coached by Art Briles. Weddell was a six-time District Coach of the Year and a Galveston County Coach of the Year on three occasions. Weddell compiled a 103–13 record at La Marque. Eleven coaches who worked with Weddell during his time at La Marque moved on to acquire head coaching jobs of their own and seven of them are still high school head coaches.
College coaching
Entering the collegiate ranks in 1998, Weddell was hired by Texas A&M University head coach R. C. Slocum to assist defensive coordinator Mike Hankwitz as middle linebackers coach. Staying in College Station until Slocum's retirement in 2002, Weddell was a part of four bowl teams and the 1998 Big 12 championship team. After a short stint as defensive coordinator at Brazoswood High School in Clute, Texas, Weddell joined Art Briles' staff at the University of Houston in 2005. Originally hired as linebackers coach, Weddell was promoted to defensive coordinator in April 2006, after Ron Harris stepped down. With Weddell at the helm, the Cougar defense allowed just 21.9 points and 339.1 yards per game during the 2006 season. After coaching staff changes when Kevin Sumlin replaced Briles as head coach, Weddell left Houston.
In 2018, Weddell was inducted into the Texas High School Coaches Association Hall of Honor. On January 15, 2019, it was announced that Weddell had accepted the position of Athletic Director of Brazosport Independent School District, which includes his alma mater Brazosport High School.
