At 9 mo, solids are indeed becoming increasingly important. However, the bottle will always remain most important up to 12 months and I believe they should be taking a min of 20 oz. If you are giving him 22-24 oz per day at 4 different feedings, I would say continue to follow that course. You can certainly add a third meal, but you may want to keep an eye out to make sure the bottle amount does not fall below 20 oz.
What are his portion sizes at a meal out of curiousity. My 8 mo DD2 normally takes 4 bottles a day (total of about 24-26 oz) and 3 meals of about 4 oz.