The forum should be fine, as long as he does not look at this specific thread:
https://powdertoy.co.uk/Discussions/Thread/View.html?Thread=2553
It kind of gets away with violating some of the rules. There are not any other threads like that though, and the mods do get ban happy if they see comment abuse of saves, so there shoudln't be any foul language or innapropriate material or anything like that.
The Forums are actually EXTREMELY clean. I have not seen one instance of harsh profanity, because the moderators are always on the watch for that kind of thing. Now with the game, there are some breakdowns in that. As long as your son continues to make his own stuff, and stay on the frontpages of the saves, he will be fine.
As long as your son doesn't act like an absolute jerk,(game or forum) he'll do fine. Nothing inapproriate really gets made in the game,and if it does,it'll get reported. Flame wars don't happen too often(at least in the forums),but if your son doesn't understand sarcasm,have a good grasp of English,or handle criticism,he'll be in a lot of trouble with some people.
Oh,and don't vote on your son's saves.In the past,we had an issue with two users who were brothers living in the same household.The mods couldn't tell if the "brother" thing was just a cover for self-voting or if it actually was his brother,so both ended up getting permabanned(although for different reasons)
i agree with jacob1 (as most people should) there are certain times where some people forget that they are playing a game that does not recquire any one to win, and so they get angry and start hating and sometimes just just downvoting for no particular reason. but as i said this does not happen often. and the forum is especially safe, anything bad on the forum's is quickly seen and taken care of by a mod.