Well here's an even more unpopular suggestion: euthanasia. It's obvious your mother can't care properly for it. You can't give it away. The dog may die of the illness. You can't spare money to take care of it, nor should you.
I know it sucks to watch the dog be in pain but your boyfriend cannot risk his job over the dog. Your baby and ya'lls well being HAS to come first. Dogs, while great creatures, aren't above human life in most instances. So it's going to have to be either the dog goes, ya'll find some other source of money (a family member or something, which you've already said wouldn't do it), or ya'll risk your baby being born into a very hard situation. It's a game of evils and there's no real way to win.
All you can do is either see if the vet will be very generous in working something out or get rid of the dog. You can't risk you and your baby on a dog. It's not logical.
Anyhow, is the blood bright red or a darkish red in color? That can help in determining where it's coming from as well (intestinal bleeding vs bladder bleeding).