Weaver was born and raised in Spokane, Washington and attended Gonzaga Preparatory School, where he played baseball and football.

He was named first-team All-State and the Inland Northwest Defensive MVP after making 123 tackles, 36 for a loss, with 14 sacks and six forced fumbles.

As a junior, Weaver started all 13 of the Golden Bears games and led the Football Bowl Subdivision with 159 tackles (9.5 for loss) with 4.5 sacks, two interceptions, and six passes broken up.

He was named the Pac-12 Conference player of the week on October 29, 2018 for an 11-tackle (one for loss) performance with a pass broken up and an interception return for a touchdown in Cal's 12-10 upset of #15 Washington.

[10][11] At the end of the season, Weaver was named the Pac-12 Defensive Player of the Year and earned first-team All-Pac-12 honors after leading the nation with a school-record and Pac-12-record 182 tackles.