There's a chance they could lean him right down and do plenty of agility work, that could make a fair difference. A frame like his could swing between 105kg at the high end to play lock, or down as low as 88 - 90kg if they wanted to maximise his agility and endurance.
It'd be a fair experiment. I think he's definitely got the ball playing, the catching/leaping and the communication ability for fullback, with his change of direction and positional play the major drawbacks, though the latter is a learned skill.
There'd be worse options. Some better ones too, but worse fullback picks no doubt.