I'm so sorry to hear this. Is there no default joint custody in your jurisdiction? In the province I practiced in, if there was no agreement, it was joint. Of course, and I mean no disrespect here, your incarceration might be a point in the mother's favour for now.

He's 5, he doesn't know now, but he's old enough to form memories. He WILL know. And his mother, if she's what you say she is, will mess up again. Document everything. Document how she's kept him from you and your wife. Write it down, times, dates, whatever.

You may not win the battle. I can't say. But in the end, you'll win the war. He'll seek you out. He might be 18 when he does it, but this kind of shit has a way of coming back on mothers like her.

I know it's little comfort. And I can only imagine how helpless you feel. Try to stay strong, don't let his mother break you. Someday, that boy will need you, and you'll need to be ready.