Есть ли среди этих предложений present simple? his achievements since his accident have shown him to be a real-life superman. they've already proved that it's possible for nerves in the spinal cords of animals to regrow and reconnect. at first, he couldn't breathe without a machine, but now he can breathe on his own.
... but now he can breathe on his own.
в этих частях ваших предложений - Present Simple.