Wa alaikum salaam
If his heart stops naturally and he chose before hand not to be resuscitated, this is up to him as this is not killing himself but rather choosing not to resuscitate.

As selam aleykum; i have a very important question. A patient is suffering from tumor. The doctor says, that it is incurable and it is probably, that this patient will have a heart attack. (The patient has still his mind, but he is very weak)Is it allowed to reanimate this patient? The doctor says, if they reanimate, the patient will live only confined on the apparatus.Barak Allah feekum