I’m so sorry your son has to feel devalued in a place that means a lot to him. I’ve worked my fair share of retail jobs and, while I’ve met some truly wonderful people, there are others I simply didn’t click with, no matter how hard I tried.
There are a lot of stressors in that environment and some folks maybe don’t have the resilience or strength of character to prevent them from taking it out on the people they work with. If it helps, let your son know that this is something a lot of us have gone through. Yeah, it sucks. :(