You have not mentioned about the age of your baby boy.
Why was he on soy formula till now? Was he having lactose intolerance?
If he had been diagnosed with lactose intolerance, he will not tolerate milk based formula and may present with loose stools.
In such a case, he has to be given soy milk or lactose free formula.
While weaning him off formula, start introducing only one food at a time, so that it will be easy for you to find out which one he does not tolerate. Every week you may introduce one new food.
If loose stools are associated with fever
etc, he might be having a gut infection- which is mostly viral in this age group. In such a case, you have to be sure that he drinks well. Continue to breast feed. You may give him boiled cooled water/ ORS/ vegetable soups/ rice based porridge/ curd etc. Be sure he wets his nappies atleast 4 to 5 times in a day.
At any time if he develops high fever/ he is dull and lethargic/ his urine frequency is less/ he does not drink well/ you see blood in stools, do not hesitate to visit your Pediatrician
Hope my answer helps you.
Wish your son good health.