My 21 week old is very fussy recently during feedings. He wont finish a full bottle anymore. He has been on gentlease since he has 8 week.?

Think of.... .... New onset of formula intolerance or a worsening of acid reflux disease.(gerd) check with your pediatrician.
Many causes. Babies can become fussy suddenly for a variety of reasons. New teeth? Fever or other illnesses? Flare up of reflux? Examine your baby carefully. If you cannot figure out the cause see your doctor to have it evaluated.