A word of caution about SLO for next years prospective students. They are overenrolled for this coming year so I would assume they will be a more difficult admit for next year. This was the first year that they did not offer Early Decision, so they probably underestimated their yield.
I would also like to mention that SLO is a very popular school for both my son’s HS. Many of my son’s friends applied to SLO for ME in particular and the majority of them were denied, however, they did end up at very good schools like UCLA/UCSD/UCSB so you can never predict any of the admissions outcome.
Apply widely for the best chances and hopefully he will have several choices from which to choose.