You could probably graduate him now from bottle? I dont know... I stopped giving bottle at 6 months. straight onto cups, then straw cups then sippy cups. Strange how we worked backwards, I dont know why we did it. So now my boy drinks from anything. He was also about 10kg at 18 months, even though he ate reasonably well and drank some milk, not sure how much as I never measured.
Also milk isn't that important after 1 year so it really doesn't matter if he isn't drinking much and it's great he takes other foods! They dont't need formula after 1 year. Cow's milk or alternatives will do