The basic problem here’s nipple confusion and hasty introduction of top milk that to by bottle.
When baby is given bottle he need nothing to do just drink where as at breast he has to latch suck and maintain pressure this makes him uncomfortable.
Henceforth slowly he stop putting effort on sucking.
At breast sucking causes reflex release of milk this is lost hence milk production decreases.
Next Steps
When hungry first put on breast later feed him top milk with container and spoon
Health Tips
Give more milk at night so that milk considerably increase in quality and quantity. Still not enough take consultation with paediatric or Gynaec doctor.