Shayne:
Good write. I like the idea that the boy is a victim of a regrettable circumstance that he can't understand yet. In the bridge, maybe you could paint the picture more clearly that it was the fault/responsibility of both Mom and Dad. I came away from the bridge feeling like Mary had walked out and was the "bad guy" in this situation. Maybe also try substituting 'mommy' for 'Mary' - that might keep it more focussed on the perspective of the boy.
Best of luck,
D