What is the so called goal with him bulking up? What does this achieve? If it's to do with breaking more tackles or making more metres...plyometric training has more to do with this rather then doing weights and being bigger. Do you want him to be more powerful? He is defensively sound, so his size doesn't restrict him there. His body can handle the rigours of first grade week in week out. Why does he need to bulk up? His body in it's current state can handle the pressures of first grade football. So what if he's 191cm and 92kg? Stats like that have nothing to do with it. I'm 186cm and 99kg and I'm by no means fat but I guarantee you Reddy would be stronger then me. If you want him being more effective in attack in terms of his hitups and see him get over the advantage line more consistently, then plyometric, power and speed training are going to achieve that, not hypertrophy (size/bulk up) training.
In his current body Joel is effective defensively, still gets offloads away. Sure he's not a huge metre eater like Grothe is but size wont achieve that. Doing plyometric, power and speed training will and I'm sure that our S&C staff have that well under control. There is no need to worry about Joel Reddy's size.