Do you own this house, or does the boyfriends mother own the house? If you own the house, call the police and animal control, if you dont' not much you can do about it.
You might want to check the laws, if that horse dies on your property, chances are you are going to have to deal with desposal. But if there are four starving horses on his property, not calling ac (if you have it in a rural area?)is just wrong. How are you going to feel if you walk out and see them dead?