Baby drank spoiled breast milk 😥

My 8 week old drank breast milk from about 8 days ago. I didn’t realize it until he had about 4 oz. Is he just going to have a tummy ache and some diarrhea? Or should I be prepared for something worse to happen? Should I stay up and keep an eye on him? (As opposed to sleeping and listening for him) he sleeps right next to me in his bassinet so I’ll hear everything he does.

(I’m sleep deprived and exhausted. I normally breast feed and my husband bottle feeds. But accidents happen)

FTM hear. Any advice will help.