Does he have any digestive problems like reflux at all? Can you post a full day for us with sleep times as well as it will give us a good picture of what is going on, so
Eat - what times, how much
Activity - how long for
Sleep - what times, how long for.
To be honest though, if he has reduced his milk intake that much I would stop solids totally and get his milk back to where it was and then slowly re-introduce solids. He doesn't need much solids, until 1 solids are for tastes and textures because breast milk or formula has all the fats and nutrients they need.
What flow teat is he on? Is he teething at all, have you tried pain meds or other teething remedy before a feed?
Laura