So sorry you are having this problem. Contact a lawyer, explain the situation and find out what your rights are as a Mother and what steps have to be taken to get your son back and if it's possible begin the process.
Tell your son you are doing this because you don't want his life to be wasted. Tell him you love him and want him be happy and to grow up having a productive life....college, a profession he'll enjoy, and maybe one day his own wife and children... Tell him even if he doesn't see it or feel it, or realize it now, you do know what is best for him and you're not going to stand by watching his life be wasted. That allowing him to live with is Dad turned out to be a huge mistake, leading to no good and you want him to come home where he'll be loved...Tell him his family at home is waiting for him and that with time everything in his life will feel a lot better.....
A boy of 13 years of age can't possible know what is best for himself. When parents divorce children tend to blame themselves for it...So he has suffered your devorce, the loss and abandonment of his Dad who left the household, your reaction and adjustment to it all, the fear of losing you as well when you started dating and remarried, then he experienced a great disappointment in his father who clearly has no interest in spending quality time with the boy....and the boy now has a false sense of feeling important and respect by a bad crowed of kids on the streets. I'd say your son has a great deal of inner issues and will need a good professional therapist to work it all out with......
I would aim for no visitation rights or at least visitation rights under supervision of the courts with his natural father due to the massive changes in the boys behavior in such a short time and getting into trouble with the law and doing poorly in school. If the boy goes back to his Dad even for weekends, he'll probably be out on the streets spending that time with this wrong crowed of kids which adds up to more trouble.
If the boy comes back to live with you, it's not going to be easy for him, you, your husband and other child. It's going to take him awhile to find his place in this family.
Besides firm ground rules, he's going to need a lot of love and support and a good therapist to work with him.
I'm hoping he comes home. Although there a bad kids in all schools, I'm hoping he can start in a new school...meet new children who are involved in hobbies, and sports and clubs at school that will hold his interests and will help in keeping him on the right road. It's all going to take time and hard work...
What ever you decide, good luck and keep us posted here...