How do you get lodgers out of your home?

My son has a mess. He used to let a friend (ex-girlfriend) live with him who had two kids. He didn't charge her rent and she and the kids had their own bedroom. She on many times has invited men to move in. Now he has this girl, her two kids, plus this man who married her and his kids when they come for the weekend living at my son’s house which is my son’s home residence. They have taken over his home. This is his house and they have taken the whole house over except for his one bedroom he sleeps in. They only give my son $75 a week and lately haven't even been giving him that to him. The bills are all in my son’s name and they don't help with utilities so they are costing him a lot more money than he benefits. How hard will this be to get them out of there? The husband pays by the week when he does pay it, but like I said, they are always behind all the time. I am concerned for him as my son is very depressed over this whole thing and doesn't know how to get out of this mess.