Although elements in cannabis have been proven to reduce the growth of brain tumors or even cause them to shrink, it is not a guaranteed cure and never do anything without the knowledge and recommendations of your brother’s doctors. Certainly do not quit the treatments they are advising.
I agree with most everything else that Latewood has said above, there is no simple easy answers we can give you.
I would recommend something strong. Something with high THC and CBD, maybe something like Amnesia Haze or Golden Leaf.
One other thing to think about, you have more control indoors and can grow all year long, you can’t really do that outdoors. I would recommend looking into designing a perpetual grow/harvest indoor grow. This way your brother will wave cannabis always available no matter the time of year.