When he was sent to the California Department of Corrections and Rehabilitation (CDCR) he was sent to a prison which acts as a "reception center." You are correct: the purpose of his stay at the reception center was to have him evaluated by both a correctional counselor as well as a mental health professional, and to observe him to see how he was adjusting to a prison environment. The purpose of that stay was to determine what kind of prison to send him to and where there is bed space.
There are different levels of custody in the prison system, from a low of Level 1 to a high of a secured housing unit (SHU). An example of the most restrictive prison in CA is Pelican Bay near the Oregon border. It's the facility where the most difficult prisoners are kept. However, most prisons have a combination of levels of custody.
Once CDCR has decided which institution to send him to, the receiving prison has to determine where, inside the institution, he will be housed. This is supposedly for his safety and to maximize his chances of a successful prison adjustment. This period should not be anywhere near 90 days.
You should know that in order to visit him in prison, you have to fill out a form which is sent to Sacramento for a clearance check. That will delay your visiting opportunities for another 4-6 weeks. Write to him right now and ask him to send you the form so you can get that process started.