Passing stool after almost every feed is very normal in the first week, especially on formula, thanks to a strong gut reflex newborns have. Straining and grunting before he goes is also common at this age, since his tummy muscles aren't yet coordinated with his bowel, and it usually settles over the next few weeks on its own.
Next Steps
Keep track that he still has plenty of wet diapers through the day and is feeding and settling well between feeds. Small, frequent formula feeds are fine until your own milk comes in.
Health Tips
Please see your pediatrician the same day if the stool turns watery with blood or mucus, he gets a fever, vomits repeatedly, or seems unusually sleepy and hard to wake.