The first thing I would do is to try to find out why your son is failing school. Is it because the work is too hard? Is he just not doing his homework, assignments or projects? Is it a social problem, like bullying that he is trying to avoid? Does he even make it to school or is he cutting class? What is he doing when he comes home from school? Is he involved in sports or other activity where that is taking up some time?
I would try talking to him first. Have a really heart to heart to see what answers you can get from him. Then I would contact the school and set up a conference. I realize you work, but you might be able to set up an early morning conference with all of his teachers. Otherwise you might have to take off a few hours to meet with the teachers when they are available - it is that important. See what the school has to say and to offer. The school may have a special program to help their students who need extra help or guidance.
If you need a tutor and money is tight, try contacting the local public library. Often they have tutoring programs that are free or low cost. Another place is the Boys and Girls club. They also sometimes offer tutoring or homework help.
Maybe your son can do the work, but just needs some organizational skills to help him when you aren't there. This might require a tutor or other adult to spend sometime with him to help him get organized and set up a routine.
Another idea is to have him talk to a family friend. Someone you both trust who will give him good, sound advice. That person could be "on call" so when you aren't there and he has a question, he can ask this other person.
These are just a few ideas, but hopefully it will get you thinking of ways to help your son.