Baby eating a lot and than stop

Kylie

My baby was eating 120ml like the usual all the time and than tonight he ate 120ml and slept for almost 5 hours and now he barely eats 100ml and only sleeping 2 hours, what should I do, I know how much he eats and for how long he is good till next feeding but this is getting stupid every 2 hours, wasting so much formula because of it, I need help he is only 8 weeks old but was eating 120ml at 3 weeks old up until now