Victoria, if you have no legal status in this child's life, it is time to change that, NOW! With bio-mom out of state and possibly have a country away at any given moment; and with hubby at work or otherwise unavailble on occasion, what would happen if you were the only available caregiver and you stepson had an accident - fell off the roof, hit by a car, had a seizure? You would not be able to approve medical treatment for him because you are not a legal guardian! The few minutes difference between you taking him to a hospital and your husband arriving there could mean the difference between life and death ... literally!
Now, the likelihood of his bio-mom signing over parental rights is probably between slim and none so you will probably only be able to obtain guardianship rights and not be able to adopt him (unless bio-mom proves to be such an unfit parent the State steps in and moves to relieve her of parental rights) But, inasmuch as you are a resident parent/guardian, you are able to establish your legal rights in your stepson's care. Regardless of anything else that may develop, this is one step you most definitely need to take.
As far as the relationship with his mother, it sounds as though this is a toxic relationship. Family Mediators will attempt to establish at the outset that one parent should NEVER speak ill of another parent and this includes step-parents as well. So bio-mom's creating a hostile environment with the child or in the child's resident home is definitely a no-no! This needs to be addressed in court as well. At this point, it appears that you need to take pre-emptive action and establish the groundwork for future visitation as well as all other issues. It sounds as though she is actually a toxic influence in your son's life and her effects on him need to be curtailed as long as she is in her present chosen lifestyle. As always, your son's wellbeing has got to be the number one consideration in all matters. Will his bio-mom's influences diminish his productivity and self-awareness in negative ways? Will her lifestyle influence him to pursue similar life choices? Does she ever appear drunk or intoxicated on drugs? These things must be considered when determining whether to allow visitation between bio-mom and your son. If she ever appears other than completely sober, you or your husband must explain to her that she will not be allowed to see her son when she is not clean and sober and, if she ever appears intoxicated, she will be turned away - by police if necesary. Make sure she understand that, for her son's sake, she cannot take him unless specific guidelines are met. And that is your husband's responsibility at this point as is the enforcement of those guidelines. And, until you can have such matters established by the court, you and your husband must present a united front against anything that may be harmful to your son - even if that is his bio-mom. If your husband is reticent to risk any confrontation with bio-mom, he needs to understand that your son's well-being and future life success (or failure) should be his first consideration.
In any case, you must act immediately to establish your parental/guardianship rights for your son's safety and security.