The issue for the reader of a new topic is to fill in blanks to the best of their ability and try to give some feedback that could be helpful. The issue for the OP (original post[er]) is to give enough information to start the discussion but not so much that you get the response "TLDR" (too long, didn't read). It's a balancing act that results in exactly what you see here. You tried to give enough information in the beginning that would get the gist across. We, naturally, had questions or made assumptions to fill in some of those blanks.
As regular posters here, many of us have natural tendencies and inclinations in how we respond. You've been introduced to some of those. Mine is to try to understand both sides of the story even though we only have one storyteller. I can see how someone in the man's position could lose his cool given the circumstances we've been given. That does NOT mean that I condone his reaction or that I even yet have enough information to see that this setting makes his anger reasonable. That he became enraged is without question. That he over reacted is also, to me, without question. I'm still trying to get a feel for if the man is simply a lunatic or if, as engineer has supposed, he felt his own safety was in peril and (over)reacted to the situation.
One detail that we don't have is the knowledge of how long the man was left standing on the tree waiting for your son to react to the requests to get down. Another one is the tone of which the requests were made.
He turned back and asked our son to get away and down to give him room to get down, too. Our son stopped but didn't get out of his way at once. He was having fun, and we had to tell him 2 more times, and then he obeyed.
If the three requests were made in a matter of a few seconds and the tone wasn't urgent then I think the man falls closer to the lunatic side of the scale. Who are the "we" that made the subsequent requests? If the kid is blowing off multiple requests from multiple adults and the tone is expressing some sense of urgency then I can see why he became enraged, particularly if he felt unsafe in any way.
Again, at this point in the discussion all of the gaps and details are moot. You came here asking about legal remedies and have decided not to pursue them (good decision, imo). Case closed from the standpoint of the OP. That the topic gives the rest of us fodder for additional discussion is simply par for the course.
BTW, welcome to A2K. Many of us came here originally with a specific question or topic of interest. The level of discussion here is what keeps many/most of us coming back.