Hi there,
I have worked with children my entire working life and I have some info for you.....
First I have to say....all babies sleep through the night when they're ready. My daughter was 10 months old before she slept through the night. There's really nothing you can do to help your son sleep longer. He'll do it on his own time.
Second, DO NOT give your son cereal in his bottle. This DOES NOT fill his tummy more AND it makes it more difficult for him to learn to eat cereal from a spoon becuase he's already getting it from a bottle. It has never been proven that cereal in a bottle fills the tummy and helps babies sleep and I would strongly advise against doing that.
Third, it is completely fine to let babies cry it out at night. Yes, they are new to the world but that's just what you have to do sometimes. It does not affect their independence. As long as they are being loved and fed and changed, their needs are being met. Crying it out does not affect them negatively. It actually helps them learn to fall asleep on their own. Something they need to start doing at about a year old.
The only thing you can do is wait. I know it's hard. I'm a single working mother and my daughter got up twice a night until she was 10 months (she's almost a year). I got up and rocked her the first time she woke up and let her cry it out the second time she woke up and she is the happiest, most independent child. It is ok to let him cry it out.
Another piece of advice....are you still feeding him when he wakes up? If you are, I would stop feeding him in the middle of the night. His body could be waking him up because it's hungry. If you get his body out of the habit of eating during the night, it won't wake him up and he might sleep longer. A five month old does not need to be eating during the night. He is old enough to sustain himself during the night. It will be an adjustment as he gets used to it but it won't take his body long to stop being hungry during the night.
I hope this helps. Good luck to you.
K.