R.R.
Hi M.,
Is it possible that he is teething? My 10-month old daughter had almost the same eating schedule as your son and was always a good eater, but when her top teeth started to come in, she stopped finishing some of her bottles and did not eat as much either until the discomfort went away. I doubt he is growing out of his bottles. Babies still need just as much formula/breastmilk as always, I believe.
Good luck!