I think worrying about "when to trust" him is just going to spin you around in circles. He will have good days and grouchy days, measuring his behavior, moods, etc, will only drive you nuts and increase stress in your life. Remember, you didn't cause it, you can't cure it. Your son needs to learn to trust HIMSELF. That is his job. He is very young, I know, but this is his burden to carry. Not yours. I think many would recommend that you stay focused on your own health, sanity, and do your best to increase the quality of your own life. That is all we can do.
Your son will do great. And so will you - just remember to step back from the addict's life and focus on your own. This will be good for both you and your son during this challenging time. All the best!