Have to say I agree with Gidget.
So long as therapy (for you BOTH) continues when you go to to Texas - then hopefully you'll be addressing the underlying issue. Simply moving away won't help. If he's got a drug problem, he'll find people to do drugs with there too. Teens can be incredibly resourceful. If he feels he knows no one and is depressed from starting over again - then you will need to have support in place to help him from the get go.
You haven't mentioned your son's father's role in his life. I am concerned uprooting him from his father may have negative consequences.
Are you getting guidance on how best to help your son? So far, it seems you've been a bit lost. No offense, I would be too - but from changing schools, pressing charges, taking him to live at dad's, to now moving - sounds very reactive. Is there a plan in place at all? He was in a psychiatric lockdown (?) wasn't he not too long ago? There should be people to help you navigate all this. I'm concerned that you're not getting guidance.
I hope this works for you - but I think unless you get the help you both need, the problems may follow you. I think not pressing charges may have been unwise. As hard as it is, sometimes a reality check (tough love) is the best thing you can do to a teen who feels out of control.
Keep us posted. I am wishing you both well.