Having him sign financial aid papers and likely enroll in classes today was a big first step. Today is the 4th day of the spring semester and the official "add/drop" end. After today, you'd need special dispensation from the registrar to enroll in a class. If you're not enrolled as a student, you're not on the team. If he signed an aid package, my guess is he's got his class schedule. He would then be an official student for the spring semester.
After this, it gets murky, and anyone who tells you they're "certain" about what's going to happen is only following their heart. USC released him from his LOI, but this literally makes zero difference. LOI's are only interesting until classes start and you're enrolled. Classes started at USC, and he was enrolled in them. He can likely un-enroll from USC without academic penalty, but (to quote U.E. McGill) the NCAA is a little more hard nosed than that.
Essentially, the NCAA considers him to be a student at USC for the spring semester the same as if he transferred in May. He will need a waiver to gain immediate eligibility, and there really aren't any comparable incidents to compare. Officially, you need to claim some sort of compelling circumstance for a waiver. Justin Fields transferred to tOSU, and is claiming that a racial incident on a baseball field caused him undue hardship at UGA. The general consensus is that this stands a good chance of satisfying the NCAA, and earning a waiver. If that's the threshold (and I'm not belittling the severity of racial abuse at all), then it seems that some unrelated incident (I wouldn't use the "my coach left" argument - it happens all the time) might be proffered to prevent 1 week from costing him an entire season.
I'm gruntled that the NCAA is well known for its rapid, fair, consistent, and common sense responses - and not being the least bit capricious and arbitrary.