Make sure the parents, you and your wife as well as the poor boy himself are fine with those arrangements.
Some questions to ponder:
Is the teenager an Orthodox Christian? Would he be willing to go to Divine Services with you? If not, Is that why he wants to be home with his dad on weekends?
If you have children, could this teenager be a bad influence on them?
The idea of this teenager spending the weekends with his dad would not be good if the dad has no desire to get off drugs. A 15 year old could also be a drug user or pusher. He could also be enabling his dad's behavior. He would have to be monitored, and the situation might turn ugly.
My own brother was/is into drugs. He would steal our possessions, lie about it , and try to blame us. My parents wanted to believe him because he could quote the Bible from memory. As a teenager of 15, he stole the probation officer's car and ran away. The authorities caught him when he was 17, and released him when he was 18 so he could serve in the Army. As an adult, he still has no morals, and thinks nothing about lying and stealing. He continues to be in denial about his drug addiction, but he believes that he will be saved because Christ died once for all. He zealously believes in "once saved, always saved" Protestantism. Lord have mercy.
Have you taken any courses on drug dependency so you would know how to deal with this problem?