12 weeks and cereal is your entire problem.
A baby cannot digest cereal at this age, whom ever told you to do that is incorrect. You are entirely causing this little baby's gas issue. The cereal is sitting for days in this little ones stomach rotting. He cannot digest it. It eventually moves on through due to the milk going in but as it rots it puts off the digestive gasses. That is most likely where the gas is coming from.
I would say if your pediatrician told you to give the baby the cereal I would run to a different doc.
A baby who has reflux is having enough issues digesting the food they are taking in, to add a carbohydrate their body can't do anything with...well, stop the cereal and you'll see a lot less problems.
The doc should give you some sort of liquid med, like Reglan or Zantac but 90% of the time if the parents will stop over feeding the baby it goes away. The med relaxes the baby's tummy muscles so they are not spasiming, the food goes out of the stomach quicker since the muscles are relaxed, the opening at the bottom of the stomach works faster and better, more food is used.
One thing we did, with the gastroenterologist full supervision and at his suggestion, was to put Mylacon Gas Drops in the bottle, then add the water, mix that up, then add the powder formula. When the boy got months older we added sometimes 16 drops to his nursery water.
The thing is that baby's don't often have actual reflux, the kind that is a muscle movement that pushes the milk back out. This shows up on an upper G I Series in the X-Ray department of a hospital. It does not only happen when food is going in, it happens all the time regardless of what the baby's mouth is doing.
Docs know that most times it is over feeding, not being held upright enough, etc...that is the culprit of the milk coming back up and out not an actual medical defect with the stomach.
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So, stop giving the baby cereal, give him smaller bottles, try to make feeding time and the time right before feeding time very calm so the baby is not sucking in a lot of air from crying, hold the baby nearly upright, keep the baby propped up after he eats for about half an hour, and see how that does.
Add the gas drops directly to the water before the powder, do not shake the bottle when you add the powder, gently swirl it. The air bubbles in the mixture also contribute to the gas.