Disagree, IMO he's just an idle <deleted> that never had any discipline.
Does the place belong to the OP? If it's rented, leave them to carry on without the ATM to pay for it.
I had a similar problem with my wife's nephews, but I had to leave to get my sanity back as it was the family house.
If the OP can't move, can't leave the GF and can't get rid of the son, just ignore it, but don't pay anything for son's expenses. It's not like he has to live in that room. Making an issue of it with GF isn't going to work out well, as he is her son, after all.