Perhaps the father is a soldier, and had already been off with the army for sometime. Perhaps the child had no father, considering the past history of orkish raids in the area. Perhaps he was just too busy preparing for his cameo as a Corsair in the next movie.
And 2000 to 10000 is pretty hellish odds, Tuor. When you have a force of 10,000 marching through your countryside, no force of 100 has any way to stand. If that was the basic unit size of Theoden's army, then the muster of Rohan was absolutely necessary. No point needlessly sacrificing men to defend an indefensible land, gather everyone to you in a central, defensible position the enemy cannot ignore. And is the 2000 figure from your encyclopedia before or after the arrival of Helm?