D
I think you have hit the nail on the head with either 2 or 3 - he was doing so well too
During the day he is hungry when you feed him on the 3hour mark or does he now play and not eat as much - if this is the case, along with this new night waking the likely culprit is the 3 hour EASY now needing to change. DS also started to do it at 3month and 1 week, although I missed the cues and just thought he was off his food. He has reduced his total daily intake from 34-35oz to 29-32oz.... this obviously contributes as well.
If it were me I would follow his eating cues for the next day or two and see how far he goes during the day, you may find he is hungrier after 3.5 or 4hrs, your new routine could then chnage to this;
age- 3 months - 1 week
weight- 19.8 lbs wow
roughly eats at 6:30, 10:30, 14:30, 18:30, 20:00, 11:00DF - it means dropping a bottle, however he should take more at each feed - you also keep your CF & DF.
Let me know how you get on.
Lauren x