It’s easiest to start with what not to do. Don’t turn the eating experience into a struggle. It’s frustrating for you and can create very negative associations around food for your child. There are changes you can make, but you need to work within his constraints.
While you are right in thinking that most six-month-olds take in more than 18 ounces of formula a day (more like 28 to 32 oz.), this is just an average and every child is different. As long as your son continues to thrive by growing and developing on target, it is best not to worry in the short term, especially since most of these food refusals are only temporary.
There are a few things you can do. First, beware of the amount of juice and water your son gets. Juice is never an essential part of a child’s diet and too much juice and/or water can reduce the amount of the more nutritious formula your son wants. If you must give him juice, limit the amount to 4 to 6 ounces a day. Second, you can add to his formula consumption by mixing it into his solid food. The cereals should be mixed with formula, and he may even tolerate it mixed in to certain fruits or vegetables. Lastly, begin to increase the variety in his solid diet by adding a new food once a week. This may give you even more opportunities to mix in some formula.
If his refusal continues for several more weeks, have him weighed and assessed at his pediatrician’s office to be sure he continues to thrive.