Don't brand yourselves as "terrible parents"; you don't deserve that label. After all, how would your other son have turned out if you hadn't made the effort to point him in the right direction? Even good parents can have rebellious children; look at Adam and Eve, whose rebellious son Cain killed his godly brother Abel (see Genesis 4:1-16).
I don't know why children from the same family can turn out so differently; in heaven we'll understand, but not now. But you need to remember that your sons chose the ways they would take. You did all you could to point them in the right direction -- but they alone are responsible for the decisions they've made and the different paths they've taken. The Bible says, "Every living soul belongs to me, the father as well as the son -- both alike belong to me. The soul who sins is the one who will die.... his blood will be on his own head" (Ezekiel 18:4,13).
Let both your sons know you love them -- and that you always will. That doesn't mean you approve of the path your one son has taken -- but in spite of that, pray for him, and let him know you care what happens to him. Most of all, pray that he will open his heart and life to Christ's transforming power.