I never did the DF with my DD, but around 8 months we dropped to 3 daily feeds, which ensured the the BT bottle was nice and big to sustain her through the night. So our feeds were 7am - 2/2:30pm - 7pm. If he's not taking a feed before bed then yes it does sound like he's just replaced the BT feed with the DF. I'd probably try to reinstate the BT one and skip/wean the DF.