As the baby gets older, his bowel movements may decrease. It is possible that the soy is not the best thing for him. Is there a reason he is on soy and not a milk-based formula? Usually it is the iron in formulas that keeps a baby from passing.
You can try giving him small amounts of prune juice to help the process. It is also possible that he is using everything he eats in energy or growth. It may be that you are not feeding him enough, in which case you should increase his intake.
He might also make up for not passing later on and have 10 in a day
It is best to talk to your doctor if he continues to not have a bowel movement.