I would find out what he is doing online, is he playing a game is he emailing , looking at news etc.
If he's playing a game ask him to cut down on game time.
I personally play an online game and they can get addictive some people have to limit their time on them. In the game I play there are some things in game that can take several hours and there are other people in theses games that are dependent on you to be there sometimes. It is hard to tell them that no you can't do this or that because it takes too long.Or that you have to go in the middle of what ever it is.
Talk to him find out what is going on. Tell him you feel he is spending too much time on the computer and that the boys and you need him. Tell him it's ok for him to be on the computer some time but not when the boys need him to do something.
I'd bet he's playing something like WoW ( world of warcraft) or another MMO ( massive multiplayer online).
Find out what he's doing then go from there.