Psychic hoods per fluff are for increasing the psykers power, and protecting the Psyker from the warp. Per this guys story, his power is at a level where the good does not increase his power, so he instead has a null rod in his spine, which purely serves to protect him from warp entity's. I'm not saying he's the best Psyker ever, far from it. He is just not able to become any stronger.
I don't want to give away too much, but he has a twin. The two share a special bond, which is the reason his power is capped out. When the two are together, his power is at its most potent. However, the more distance between the two, and his twin becomes stronger. His twin is actualy the stronger of the two, except for when they are together.
He's also a throw back to when the hoods where optional