In fall of the reach, after the implants, almost half of the Spartans-in-training died, some lived but were seriously injured, and left permanently disabled. That easily could’ve happened to him as well, he was pretty lucky to survived. Hell, when John asks Dr. Halsey how to win(he thinks the biological implants is one of Chief Mendez’s tests), She says, and I quote,
“I’ll tell you how to win, John,”
“You have to survive.”