My brother has brain tumor. What should I tell him?

Brain tumor. Tell him that you are completely there for him no matter what happens. Then help him obtain as much information about brain tumors, the different types, the different treatments and outcomes, as you can. Since you are clearly good at using the internet, you should be able to get all the information you need there. This will enable him to have informed conversations with his doctors about treatment.
Truth. Tell him the truth. Develope a treatment plan with your oncologist and neurosurgeon.