It does seem like he's drinking quite little for a 6 month old. My boy's average is 180-210ml per feed, every 3-4hrs. Mix fed BM and FM, and Cerelac or puree meal 2-3 times a day.
Perhaps, you may want to try upgrading the level of his bottle teat. My 6mo boy too recently drank less than his usual but aft changing, he's back to drinking his normal amt. Also, it wld be good to start solids for him, it'll help him to feel full longer. My baby no longer wakes up multiple times at night and wld usually slp frm 11pm to 5 or 6am straight since 5 months when I started solids for him.