My son does understand and have language to discuss feelings and how they affect our behavior.
From your OP, it sounds like your mother's primary symptoms are around mania/agitation?
"brain doesn't feel well" is a little more high-concept than I think you might want at this age. If he understands feelings and behavior, think only in terms of feelings and behavior. I don't know what his translation for 'edgy' or 'wound-up" would be - but telling him that "granny has a kind of illness that makes her feel wound-up all the time, and she can't make it stop on her own. So, she's trying different kinds of medicine to help her."
I'm assuming that there's a notion of 'wound-up' (or however you call it in your household) that he will know how to equate with being tired, irritable, tearful, and lashing out. Even better if he understands that loud noises or too much stuff happening, when he's wound-up, make him feel worse and behave worse.
I'm also assuming that he knows it's not a good feeling. The idea that she can't make it stop may be a little scary for him, but the medicine is an important element that balances that.
It's important that he identify with how grandma feels, that the feeling is unpleasant, that it's not anyone's fault, that there's medicine. Things that are about "until she feels better" create a future point that is unnecessarily anxiety-inducing. Keep the whole conversation in the now. If it gets to the future - "will she be OK" you can be honest with saying that you hope so and believe so. If you are religious he can add her to prayers - or send her good thoughts, or whatever is appropriate in your cosmology.
Questions about 'why' won't be hard - either you remind him about why he gets wound-up (all sorts of reasons), and you can tell him as a fact that people can get 'stuck' in their feelings. He probably has an experience you can identify with this. Keep coming back to how he would experience it, and how he can understand that having alone-time is really just the natural, best thing.
Because, it actually is. This is all legit. It's about informing him from an empathetic (identification) stance, instead of an observatory stance.
If your mom is completely refusing medication, then you'll need to use whatever she may be trying, or could be trying. Eat good food, get enough sleep, exercise - these are all things that can help her get un-stuck from feeling wound-up.
I hope this helps, some.