The passage is "Gabriel, wrapped in his inadequate blanket, was hunched, shivering, and silent in his little seat.
Jonas stopped the bike wearily, lifted the child down, and realized with heartbreak how cold and weak Gabe had become. Standing in the freezing mound that was thickening around his numb feet, Jonas opened his own tunic, held Gabriel to his bare chest"
So he wants to share his body heat with Gabriel.