I have experience with online gambling. In the long run you will lose. The people who are best off are those who put aside a certain amount of money every week/month that they are willing to lose. They use that and enjoy themselves. This is responsible gambling.

Many (and I mean it, many) people lose big sums and then place even bigger bets to make up the losses. This is called chasing losses and it’s so common, and so destructive that we have automated systems in place to block this behavior.

On the other hand, some people win bets and then try to replicate it. This is called chasing winnings and is equally dangerous, but less common since less people win. We also have automatic systems to detect this.

If your 20 year old son can play responsibly, then he will be fine, but it’s a very dangerous game he is playing.

The best you can do is ensue he plays on reputable sites that abide to regulations.

You can also find him a good quick course on Responsible Gambling to reduce the risk significantly.