OMG!!! After so many sippy cups that I have tried my 1 year old wouldn't?

drink milk from a sippy every else was fine except milk. I had bought playtex, straw etc. I use the avent bottles and had one avent sippy cup the blue one and I never realized that I could put that spout onto the bottle ( I know dumb me) So tonight I tried it and he only drank 4 oz but still left 4 more, but never has he drank milk from a sippy. Any stories and when was your bb off the bottle, from now on no more bottle I will continue to give it to him like this!!! I hope it doesn't change