Have you tried a faster flow teat to see if that helps him drink more? Does he show any signs of digestive discomfort like reflux
http://babywhispererforums.com/index.php?topic=654.0 that may be a long shot but trouble with feeding can be a clue of something going on.
With the rusk I would really advise against it. There are a few reasons really - the guidelines are 17 weeks for solids at the earliest and then under medical supervision (my DD started then because of reflux). Solids are hard on immature digestive systems so you may be making things worse, it will also fill the space in his tummy that is needed for the more calorie and nutrient dense milk. You need to watch with rusks as they have gluten which shouldn't be given before 6 months and even the gluten free ones are full of other ingredients and sugar. Rice is usually a first food because it is simple - if there are any reactions with multiple ingredients you wouldn't know the cause.
I'm not sure the hungry baby milk will help either, especially if he's not drinking as much as he was. Hungry baby milk is designed to be harder to digest for babies who are drinking huge amounts of formula (40oz+) but are too young for weaning so it's cutting their calories by making them less hungry.
I'm assuming bedtime and last feed is around 7? So he actually went from 7-7 without a feed last night - that is excellent for a 12 week old. Even going from 7-3 (8 hours!) is really good! Sleeping through is as much developmental as calorie driven, so only 1 feed without a dreamfeed is something a lot of babies don't do until a lot older than your LO. My two never took to the DF so we just left them to wake when they were hungry (both refluxers), DS had a feed until he was 1 and we weaned DD's nightfeed around 8.5 months.
It looks like you are feeding around 4 hours? At 12 weeks they should really be on a 3 hour EASY although formula fed babies do move to 4 hours before breast fed babies. Can you post your whole day in terms of
Eat - what times how much
Activity - how long for
Sleep - what times, how long for
Laura