Congratulations to your son. My daughter had played in 12 megafinals and I am a big supporter.
The main thing he should understand is "Do not fiddle with your pieces". This is the Touch Move rule. If you touch your own piece, then you have to move it. If you touch your opponent's piece, then you have to take it (if you can legally make that move). These are the biggest areas where new players fall down and many megafinal organisers will be watching out for new players nervously selecting which piece to move. In our local one we take the view that most will not fully appreciate the importance and we give them a reminder or two in the early rounds. This rule then moves onto what happens when you are in check. Clearly, if you have been touching loads of pieces before making each move, when you get to the check and the first piece you touch is your Queen you may find that you have to use the Queen to block the check and then the checking piece (maybe a lowly Bishop) can take it. Young players get really upset by this. So, it all comes back to only touching the piece when you are ready to move it.
You will see other posts here about how Illegal moves lose instantly. I am not a fan of this rule and maybe some megafinals do not enforce this on the basis that it penalises inexperienced players. You would need to check. An illegal move is where eg a player is in check but makes a different move to one that is needed to get out of check. Or, another example, trying to move a Knight like a Bishop. Commonsense by the organisers should be the order here to avoid upsets.
The other issue new children have is using clocks for the first time. The main things they do is forget to press the button to start the opponent's time. They very quickly get the hang of this. The second issue is thinking they have to play fast because they are running out of time. That is a bigger problem but they will learn as they play more games.
In the end, this is the BEST competition to start children in the world of competitive chess. The key is to enjoy it, warts and all. Good luck.