While it should be noted that if your son (or anyone for that matter) really wants to do something, they will, I will address some common concerns that one might have with getting into CS:GO.
Impact of violent games like CS on real world violence: Their is no evidence to suggest that playing violent games increases likelihood of real world violence.Time management: Ensure that your son keeps track of time and does not play for longer than is warranted. A good cutoff is no more than 2–3 hours a day or 14–21 hours a week. Matches can take a varying amount of time, usually between 30 minutes to an hour, which may cause issues if the schedule is strict.Behaviour: Since players don't interact face to face, they may be less considerate and have little fear of repercussion for their bad behaviour. Ensure your son does not behave badly with other players as well.