Babies go through many phases as they grow, their sleep patterns change constantly. There are many reasons a baby may wake at night. Do you breastfeed him, if so, remember the breast is a mothering tool (comfort, security, food, drink etc) Babies need the closeness of their mothers at night. Teething, wind/gas, hot/cold, growth spurt, developmental milestone & even a wonder week(there are 10 fussy periods that babies go through), Illness & the big culprit - starting solids too early, especially cereal
Just as you can't know when your baby will first smile or start to talk or decide to give up morning naps, there's no way to predict when your baby will sleep through the night. More importantly, there's no "right age" at which your baby should. As with any developmental milestone, your baby may be earlier or later than other babies in developing new skills. Even if they did sleep through previously, it doesn't mean it's going to last long. Things change.....
In the early weeks, remember that your baby may not get enough nourishment if he sleeps through the night. Breastfed babies need to breastfeed at least eight to twelve times every 24 hours, usually every two to three hours. Most babies will gradually sleep for longer stretches at night, but they will continue to need night feedings for months.
You may have heard that giving your baby cereal will encourage sleeping longer at night. This simply isn't true. A baby's immature digestive system isn't ready for solid food until some time around the middle of the first year, and solids given too early may actually upset a baby's tummy.
Many mothers find that keeping baby close all night makes night timeparenting less tiring. Throughout history, babies and mothers have traditionally slept close to each other. The idea that a baby belongs all alone in a crib is a fairly recent notion. Many have found "shared sleeping" or "the family bed" a good way to meet babies' nighttime needs with few interruptions to the parents' sleep. Rolling over to nurse your baby and drifting peacefully back to sleep is so much easier (and warmer in the winter!) than getting out of bed, going to the crib, sitting up to breastfeed the baby, and then struggling to get both of you back to sleep. Not sure if you are bf once again

or if you are already co-sleeping but this might be a temporary solution.
Some authorities believe that parents need to teach babies to comfort themselves when they awaken at night, and some go as far as suggesting how long parents can allow a child to "cry it out" before responding. While such methods may work for some families, many other mothers and fathers have found peace in trusting their instincts and responding to their babies' cries. It helps to remember that babies' sleep cycles are very different from those of adults, and a young infant needs to awaken during the night in order to get enough nourishment.
Sorry for the long post, I tend to get carried away

Hope this will help in some way....
I went back to work when my son was 5 months old. I stressed about sleep & if I'm going to cope at work but I'm perfectly fine. I still bf him. He is a real boobie monster but we co-sleep so when he just cries for milkies through the night, I just lift my top & sleep further. He doesn't really bother me at night.