It could be that ds is filling up on solid foods so is not hungry for his bedtime bottle... at 6 months milk is still the primary nutrition for babies, solid foods are for learning what foods are and what to do with them, so that by the time they are needed for nutrition (around 12 months, when milk is insufficient) baby is confident about eating.
Even tho ds is a big boy, you can afford to cut back the amount of solid foods you offer because they have relatively few of the calories or fats needed for his development and to satisfy his hunger, the milk is what he needs, so I'd try this first to see if it encourages him to take his final feed!
Ax