i would kind of let him have some space..this is probably freaking him out a bit. being a kid is hard enough but i am sure this whole thing was a real shock to him and he needs time to get through all this..take it slow.
Shelley, I don't think you're gutless. And I'd be scared, too.
If you have his adoptive parents permission, then I'd call but not expect a real conversation. Just a "Hi, how are you?" kind of thing.
If it were me, I'd ask him if you can write to him. And then I'd write him exactly what you've told us through your posts. That you loved him & his brother but that you were too ill to give them what they needed. If he's coping with bi-polar issues, then he may understand better than you realize. If he was 6, he has to remember getting in trouble with the school and such.
Shelley, please don't wait. Even if you just call and leave a message that you called to say 'Hi!'. Early teen years are a rough patch of life. The thought that you tried to call may make more difference than you'll ever know.
Even if you call and don't get much response from him, you've opened the door. My eldest brother had a son when he was 18, and (long story) lost touch with him. The boy's mother contacted my brother when the son was about 12 years old, and they met. No other contact for several years, but they reunited when the son was in his twenties, and better able to understand the circumstances, and they see each other quite frequently, now.