It may depend on time of day too. My baby will drink more/longer (I breastfeed and pump) right before bed and then will sleep longer than usual.
Mine is inconsistent as well. We just feed her when she wants it. And if she doesn't finish a bottle, no big deal. As long as they are eating and having dirty diapers, all is good! You're doing great!
I would just do 2oz. Like each time (: your baby will let you know if he's still hungry (:. You are doing great mama ❣️ also with mine I burp him and then sometimes that helps him finish lol. When in doubt ask his pediatrician