Answer:
His molars are coming in, and he is teething
Step-by-step explanation:
Early Formation of molars and Teeth tend to occurs between 12-18 months of age. For infants, this process create uncomfortable feeling in their mouth or even a painful sensation.
This pain is what constantly wake the infant up and cry in the middle of the night. The pain will also explain why the infant is acting really fussy during the day. It heavily affect their mood but the infant hasn't develop enough ability to communicate the problem to the parents.