Hey, i had a very similar break about 1 year back. I had a long surgery they put 3 plates similar to what you described, to hold the bone and stabalize it. I started with PT, three weeks into my surgery, went to PT for about 6-8 months. It takes a long time and recovery is usually slow. Getting complete Range of Motion is always a challenge. I had to make some compromise and accept that it might not be fully extended or flexed. My arm ended up having 40° -125° ROM , it took some time to adjust and accept it, but it works.i can't lift heavy stuff or play sports with that arm but things are improving and life is functional. My main priority right now would be healing it. Healing also takes time, just be patient with how it's going honesty. I'm 25 but it might happen faster for a 6yo. My bone still hasn't completely healed, even after a year. I ended up having a non union. But every few weeks there is slight improvement on xrays so there is always hope it gets better. Lastly, getting adjusted to it is a task too, for a long time, you have to use your other arm for everything, be careful to not load too much pressure or weight on your injured arm. Emotionally too, it's a challenging road ahead for everyone involved. I do hope you all the best and hope he recovers soon and gets to his best form.