Since i've had the stomach flu with diarrhea and i'm nursing the baby, will he get sick?

Likely: I would say it is likely but hard to be sure, especially if you were very diligent in washing your hands.

No: Whenever someone in your house is sick there's always a chance your baby may get sick too. But whether it's you, dad or a sibling who's sick, the best thing you can do to protect your baby is breastfeed. After exposure to a virus your body will make proteins that fight the virus. Those proteins pass into your breastmilk and protect your baby. So wash your hands well and keep on breastfeeding.