I'm not trying to be silly in what I say but the next time when your mother and that guy are alone in a room, you should just burst in pretending you're looking for something. And if you catch them doing the haram then tell the guy to leave your house or you and your other brothers will physically hurt him. He'll gey scared and won't come back inshallah. But I don't know what you can say to your mum. It depends on what her reaction will be when she knows that you and your siblings found out.
If you are not man enough to do the above (I don't mean to say this in a mean way btw) when the guy comes and say he's in the kitchen, living room etc. with your mum, then sit with them and keep asking him when he's going to leave. Indicate that you don't like him being with your mum. Maybe even ask him would he mind if you told his wife and children that he comes over your house to spend time with your mum.
I hope Allah makes things easy for you inshallah.
If you are not man enough to do the above (I don't mean to say this in a mean way btw) when the guy comes and say he's in the kitchen, living room etc. with your mum, then sit with them and keep asking him when he's going to leave. Indicate that you don't like him being with your mum. Maybe even ask him would he mind if you told his wife and children that he comes over your house to spend time with your mum.
I hope Allah makes things easy for you inshallah.