If your kids are already noticing the path towards possible divorce, then you must immediately start telling your kids that they have a loving mother and father, and divorce or no divorce, that they will continue having parents who love them. Keep reminding them of this.
If he really wants to change, encourage him to seek therapy on his own. Also try to see how the internet usage can be limited? Can you password his device? Unplug the router? Is he so addicted he uses it on shabbos? If he doesn’t use it on shabbos show him how he is able to let go.
Why is he on the internet? Is this a cause or an effect? Does he rum to the internet cause of his addiction , or is it his drug, his escape? If so what is he escaping? If the latter is true, the internet might not be the main issue, it might go away if what is sending him to the net disappears.
Good luck