Your question about how to help babies strike a balance between eating solid foods and drinking is an extremely common one, and one that definitely has the potential to pose several challenges. Fortunately, these challenges tend to be short-lived, once you are able to find a happy medium. For some babies (and parents), it’s a question of timing and figuring out when to interject solid food feedings into the presumably well-established drinking schedule. For others, it’s a matter of preference, as some babies are far more interested in one than the other (whether that is a disinterest in solid foods or a dominating preference for them). In any case, it’s best to have the following guidelines in mind as you introduce your baby to solid foods.
As you noted in your son’s case, it is important to be aware that what babies drink (whether it is formula or breast milk) contributes significantly to their daily nutritional intake during the first year. For those babies who suddenly shun the thought of drinking as soon as they are introduced to solid foods, I’ve found the trick is often to simply shift the schedule—much like you would with older children whose dinner necessarily comes before dessert—such that liquids come before solids. The other thing that is often helpful is simply pointing out that the total ounces of formula (or breast milk) used to mix baby cereal can and should be counted in the total number of ounces a baby drinks in any given day. For babies like your son who seem particularly partial to solid foods, the additional 4 (or even 6+) ounces served mixed in with baby cereal can help make up the difference. While the suggested amount of formula for 6-month-olds is typically on the order of 24 to 32 ounces, do be aware that this range will vary depending on whether solid foods have being introduced yet.
And that leads me to point out what I think is most important to emphasize: the helpful role your pediatrician can and should play in helping you and your son ease into a healthy new and customized feeding routine. This will necessarily involve taking into account your baby’s rate of growth and development, as babies who are barely keeping up on their growth curves lend themselves to a very different conversation than those who are doing a good job of following a predicted pattern of growth.
I hope that helps, and I wish you all the best in your feeding adventures.