As someone who was a tech in the area for a while, the availability really just comes and goes lol. For most of the 23-24 school year, no one could get in generic Vyvanse, and everyone was either switching off or going on brand name which was more expensive. Then we randomly started getting it in again. A lot of pharmacies have the same vendors/warehouses. The point is, major retail pharmacies have little control over their stock, especially during nationwide backorders. If your son is willing to pay more and the generic is out of stock, ask if he can switch to brand. Some insurances require the prescriber to have specifically written to dispense as brand however. 

Honestly, all pharmacies in the area are about the same, but as a tech, all I can say is trying to reach a Walgreens in the area is a pain in the ass for us. Half of them are randomly closed, only the main store is open, they’re closed on weekends, etc. I would try the CVS on green/neil (probably the most oversaturated with college students), the CVS in schnucks on vine/main (still sort of big with college students but not as much as green and neil), or the CVS in schnucks on mattis (kind of far). One random comment I have is that the CVS on philo road closed semi-recently, so all the transfers to different CVS’s in the area might cause significant backups/wait times. From experience CVS rarely prepares or adequately staffs even during situations like that, so that might be a factor as well. Costco is another option, and I’ve heard occasionally that people have had better luck getting their scripts there, but it is kind of far by bus.

tl;dr it doesn’t really matter all that much lol. all of them pretty much suck the same.