Regarding GPA, yes SLO is the exception for GPA which uses 9-11th grades for the a-g courses while the rest of the CSU’s and UC’s only use 10-11th grades which could be an advantage for a student with an increasing GPA trend for 10-11th grades. Your son does not need to go OOS for a good ME program. I am a big fan of the Cal states and if his Capped weighted CSU/UC GPA is in the 3.5+ range, there are several good options. Take a look at the less impacted CSU campuses for Engineering.
https://www2.calstate.edu/attend/impaction-at-the-csu/Documents/ImpactedProgramsMatrix.pdf