My 6 day old boy seems to be having a runny tummy. i have been breast feeding, and also feeding him formula in between when my breasts don't have enough milk. could this be the cause for his runny tummy?

Immediate exam!: It's possible the formula you are using in between breast feeding is causing your baby to have loose bowels. Your baby could also have an infection or some other illness. In a baby this young I suggest an immediate well baby check at a medical clinic or Urgent Care Center to be on the safe side. I hope for the best for the health of your baby!