Never! There is no reason to use cereal if you wait until the recommended 6 month mark to start solids. Size and weight have NOTHING to do with readiness for solids, only time makes a baby's "open gut" close and at that point they are ready for solids. Cereal is nothing but stripped down grain with a few nutrients added back in. It's meant to "teach" 4-month old babies how to eat because they aren't ready yet and their natural tongue reflex pushes the food out over and over until they learn how to over come it - or you can just wait until 6 months and the reflex goes away on it's own! Babies need nothing but breastmilk or formula for the first year. Solids are started at 6 months or later only to teach babies about new tastes and textures and ready them for when they get most of their nutrition from solids after a year.
You NEVER NEVER NEVER put cereal in a bottle. It's not good for their digestive systems to get it too early and can lead to stomach problems and allergies for the rest of their lives. My husband is a perfect example of this. His parents were told that because he was a "big baby" he NEEDED cereal at 6 weeks. He still has digestive issues to this day.
Up the amount of formula you feed if you have to, but don't start solids too early, you will be harming your son. Cereal is undigestable and just sits in the stomach taking up space so you are actually STARVING a baby when you start solids early because their body thinks they are full but they aren't getting any real nutrients from food. Some babies can go longer in between meals with solids, but if they are started at the right time, most don't go longer. A good indication that your child isn't ready for solids is if they all of a sudden sleep a long time or don't need to be fed again for quite a while. It means they haven't digested the food you gave them.
Here is what you are waiting for (babies need to attain the first 3, the weight depends on size at birth):
~6 months old
~can sit unassisted in a highchair and eat from a spoon or by hand (reclined in a highchair doesn't count, they must be sitting up)
~shows interest in your food when sitting at the family meal (pretends to chew, plays with a spoon, watches you put food in your mouth etc...)
~doubled birth weight
Good Luck:) You've still got 3 months of big bottles before you should add any solids.
P.S. Another option? Relactate and breastfeed in between bottles so he's not getting so many chemicals and you don't have to spend so much.