I hate to tell you this, but once you terminated your parental rights and agreed to allow the child to be adopted, all your legal rights and obligations to him were severed. As such, you couldn't get any type of custody because you are legally unrelated to the child even though he is your biological son.
When someone is legally adopted, it cuts off any legal relationship with the biological parent. So you would have no legal standing to petition the courts for any custody. However, when the child turns 18, you could legally adopt him with his consent since he would be an adult. But at 18 he could live where he wants to anyway, so it wouldn't make a real difference.
So at this point, mother and step father could legally compel him to return because he is a minor by filing a police report with him as a runaway. If law enforcement found out where he is they could take him into custody and return him to them. But the question is whether they would actually do so as he would likely just run away again.
I know this is not the news you had hoped for.
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