Firstly, why did going deaf also remove HW's ability to speak? Not speak correctly or even coherently, but use his vocal cords at all?
Secondly, what was his purpose for trying to burn down the shack, presumably to kill his adopted father and his adopted father's brother? He apparently has presence of mind to set up a relatively elaborate fire trap, but not talk or even gesture to his adopted father. I presume he wants both of them dead, because there are far easier and more straightforward ways to kill one or the other. If it was something he saw in the photo, then it's the same deal: why not just show him the photo instead of trying to kill both of them?