There's a fine line between being understanding and enabling.

I'm bipolar and there are many times I feel it makes it harder to "move". Especially when I'm depressed I just feel stuck. I know I need to do X but the effort to do it seems so monumental. However; it's still my decision good or bad. It's still your son's decision to turn himself in. Being bipolar does not make you incapable of good decisions it just seems to be a little harder to get off your duff and do it.