So, uh, he's still got his SPARTAN armour on, right? Let's think.
His Forerunner prosthetics start to infuse with his armour, somehow spreading through it or something. Upon seeing this, Jay just thinks he's hallucinating cause he's seeing glpyhs start to appear and disappear all over his armour. But, out of curiousity, he touches the one that remains still, sitting there on his arm. It starts to pulsate, and then the proper transformation begins, eventually his armour's all blue and black, and his fatal wound starts to heal.
Just off the top of my head...