Grandma isn’t just letting him [b]have[/b] the money, and he’s attending out of her preferences and having attained scholarships as well. It’s all a deliberate design to develop the ‘nice guy’ mental state. Weak father figure, raised by a singular ‘mother’ entity, relied on by others for emotional support, having to earn everything in transactions, and then a (subtle) sense of abandonment instilled by being drip fed the money and thrown to the wolves (so to speak) by living alone.

It’s a complex mental state that I pretty much just followed the textbook recipe to develop with his background and circumstances. Having enough money to do whatever he wants but feeling like it might all go away if he makes a mistake is a huge stressor. So, to answer you directly, in a way he is ‘slumming’ it because, and he doesn’t realize this overtly, he’s worried that he’ll end up cut off if he displeases the family matriarch.

In short, he has money but not ‘fuck you’ money and though he could live beyond his current means comfortably he has a fear that it will all go away without anything he can do about it.