What patch are you using? In the original game, forces had to fight their way out of a situation like you describe. This was patched to where the trapped force would escape. This patch caused some CSA armies to move toward Canada in an endless running battle in single player. If he still wants to play, I advise breaking his unit down into small stealthy groups of 1-4 elements and slipping across the Potomac in Green/Green mode.
I'm the 51st shade of gray. Eat, pray, Charge!