There does come a time when the most important thing you can for someone is to keep them as comfortable as possible, to take away their pain and their anxiety, and to be with them. At that point, trying to aggressively treat their HIV disease or its complications may be futile and may only add to their misery. Because I don't know him, I can't say whether your brother has reached that point yet. We know that even people with very low CD4 counts and advanced disease can sometimes have dramatic responses to aggressive antiretroviral therapy with protease inhibitors. However, those therapies can be complicated and can have a lot of side effects, especially in patients who are very sick.
Some people are simply too sick to either tolerate the therapy or to get much benefit from it.
I can't tell you how your brother will die. Yes, the heart stops, but that is not usually the cause of death, simply the result. People can die of PCP, but there are many other complications that can lead to death. If you feel that that it's too early to be giving up and that your brother may benefit from more therapy than his doctor wants to give, you might look for a second opinion. I have found that doctors who are not experienced with HIV infection sometimes assume that things are irreversible long before they actually are.