J.,
My boy is now 6 months old and has been sleeping through the night for 2.5 months. When our pediatrician first suggested that we start trying to get him to sleep through the night he also provided a few tactics. Our pediatrician had us giving our boy 1 oz. of Pedialyte when he would wake at night. Eventually, he would stop waking in the night. Prior to his bedtime, try to "cluster feed." That is, feed your baby more frequently before bedtime so that there is no need to feed during the night. So, if your original schedule is every 3.5 hrs, then around 4 or 5 pm, start feeding your baby every 2 hrs until bedtime. It allows the baby to eat enough food to stave off hunger during nighttime.
Baby may also be waking for other reasons besides hunger. If your baby isn't getting enough attention from mom during the day, for instance. Your baby maybe trying to get it during the evening to stay asleep. At this age, your baby KNOWS how to get if your baby needs it.
However, my best advice...seek advice from your pediatrician who may have ideas that would help your little one. Your pediatrician knows your baby best.
Hope this helps!
K