Although the temperature at altitude can be several thousands of degrees, the atmosphere is so thin it does not transfer heat efficiently. Wikipedia explains it very well -
The highly diluted gas in this layer can reach 2,500 °C (4,530 °F) during the day. Even though the temperature is so high, one would not feel warm in the thermosphere, because it is so near vacuum that there is not enough contact with the few atoms of gas to transfer much heat. A normal thermometer might indicate significantly below 0 °C (32 °F), at least at night, because the energy lost by thermal radiation would exceed the energy acquired from the atmospheric gas by direct contact.
The high temperatures experienced by those travelling out of and back into the atmosphere are those of greatest concern, particularly the latter. On ascent, temperatures are high but comfortably within the parameters of modern materials because the craft are travelling relatively slowly in the thicker atmosphere and faster when the atmosphere thins/becomes vacuum. On descent, i.e. re-entry, the shock wave caused by spacecraft colliding with atoms in the air causes the air to compress, which in turn creates heat. It is not due to friction. If you want to see the same effect on a smaller scale, take a deflated football and pump it up as fast as you can, then quickly feel the nozzle of your pump. It will be hot, because the compression of air causing heat has transferred to the nozzle.
Flat earth is an unfortunate belief at odds with mountains of evidence. Sorry to hear your brother has fallen for it.
I suspect that even providing evidence will not be sufficient to dissuade him, based on my own experience of flat-earthers.