Difficulty in feeding

Hello mummies, my LO one is 11 weeks old and his daily milk intake has dropped significantly suddenly. He is 4.9kg, and is usually taking in about 800-900ml (80-120ml per feed) of expressed breast milk daily. However, it has been dropping since 3 days ago (about 670ml, 50-100ml per feed). Feeding him has been very tough as he kept refusing to drink it/fall asleep while drinking. He is still having at least 6 wet diapers but hasn’t poop since yesterday. Any tips?