Too soon to say. Players move positions all the time very few players look to be able play several positions.
He could get a start at a few clubs now in the halves. His earning potential is highest as a half, why would he bulk up and try to become a lock or backrower? So he can earn less money and still be outside the top 17 here?. Its not going to happen. He might hang around for a few seasons playing out of position when he can but he will still be trying to get a starting spot in our halves.
Unless he is still growing(which is unlikely) he isnt going to put on 6 kilos in an off season again unless he really is trying, and even then he will struggle to put on that much muscle keep his cardiovascular fitness up. Putting on 6 kilos of muscle while doing as much fitness work as an NRL player is really f**king hard. Its not like they can just eat and lift, even doing just that it takes a couple months and a lot of hard work and discipline to put on muscle without putting on fat. If he wants to be a half he will just keep his weight down.