Sarawak, for as long as your son smokes only socially, doesn't get into the habit of smoking it in the morning, and stays interested and engaged in the rest of his life, I don't think you need to worry.
IME there are two different types of cannabis smokers: those who smoke it to wind down after busy days/weeks, and those who smoke it because they have not enough else to do and/or to switch off and opt out from lives they don't like. The first group tend to use cannabis without it causing them any problems...
Apologies in advance for the rest of this post. It's long and potentially boring for anyone not interested in the chemistry of cannabis...
There is a very important difference between skunk and other forms of cannabis, which is very poorly understood. It is important because it means skunk is potentially dangerous, while other forms of cannabis are not. It explains why views about cannabis often polarise, because most people don't understand the difference.
Cannabis has two main 'ingredients': THC and cannabidiol/CBD. The THC is the main psychoactive component, affecting the mind most and giving most of the 'high'. CBD is less psychoactive, has proven medical benefits (see link) and appears to reduce or mitigate some of the more powerful effects of THC. Particularly, and crucially for experimenting teenagers and their parents, THC appears to have a psychotic effect on some people, but CBD appears to have a 'balancing' anti-psychotic effect.
In 'normal' cannabis, CBD makes up about 40% of the drug and THC a much smaller (but still powerful) 3-4%. However, in skunk, the plant is genetically modified to add extra THC for more of a high, by reducing the amount of CBD (they alter the drug's molecular structure). So, skunk producers are aiming to increase the psychoactive element, but are also incidentally increasing its psychotic element, and simultaneously reducing its anti-psychotic, 'protective' element.
That means that skunk is much more variable and unpredictable, and potentially much more dangerous, than other forms of cannabis.
You will still get people telling you that cannabis is 'harmless'. Generally they aren't clear about the difference between skunk and other forms. Very many parents of teenagers smoking skunk - including me - will tell you it causes problems, including erratic, aggressive, depressive, paranoid, and psychotic or near-psychotic behaviour. IME, other types of cannabis do not do this.
If you want a graphic example of the very different effects of skunk and 'normal' cannabis, watch this video or the BBC3 documentary it's extracted from.
UK police estimate that over 70% of seized cannabis is now skunk. It's increasingly difficult to get hold of non-skunk cannabis in the uk. That's very bad news, as far as I'm concerned...
More wiki info about cannabis here. It's also worth looking it up on Frank.