My five month old only will eat when he drowsy or sleep he has been on reflux medication for awhile and he doesn't have big appetite what could it be?

Normal feeding? You mention reflux medication and i see on the profile "hydrochlorothiazide." i assume that's your medication and not your baby's. If he is breastfeeding or formula feeding well and is gaining weight, then there is no concern. I was not clear from your question if you meant he was not having a big appetite for solids or for his milk?