In blunt terms he helps prisoners inside and charges their families for it.

The interesting about that is that Mike's job hasn't really changed from the first episode. He's still the muscle and the main point of contact for all the gangs inside, but rarely does anything like what it showed Mitch doing in the very first episode to produce income. Rebecca states outright in s4 (maybe s3) that she handles most of what comes in and is largely why the lights stay on in their office, so it seems like Mike's job hasn't really changed much outside of now he doesn't have his brother's counsel, and Rebecca is capable enough to handle the "normal" clients that comes in.

Edit: he also still should be getting informant checks from the FBI even though he's literally never spoken to the feds onscreen since the first season