K.M.
Definitely agree with SH about checking with the pediatrician. Sippy cups are fun to chew on, but usually they are closer to a year before they can drink much from them.
Be sure to give the bottle first, then supplement with food. I often gave solid foods at breakfast time and dinner - dinner meant that my daughter went to bed nice and full - a good bottle and food held her longer so she slept longer.
Also, how long has been taking less? It could very well be a "stage" - they constantly change - sometimes my daughter eats a ton, other times she is like a bird. Good luck - just keep offering and check with your doctor!