This might sound cruel, but I used vinegar. My son was over 2 years old and still breast feeding. My body was falling apart under the strain and I had to stop. There is medication that I am mean to take that I put off for over a year to continue breastfeeding and I HAD to start taking it again. Following a suggestion from a doctor friend, I put vinegary water on my nipple. He only tasted it twice and that was enough. The first time he made a face and looked really surprised, so I did too, and I asked him what was wrong. He said 'Booah is ucky'. and I replied, 'Awww... is it? That happens sometimes when you get big'. and I could be very sympathetic with him and tell him what a big boy he was getting to be and it was sad that Booah tasted icky now.... I had to dab a bit on for a while so he could smell it when he lifted my shirt and that's all it took. He came up to me a few times and lifted my shirt and pointed and said 'Booah Ucky!' Maybe you could 'flavour' his bottle nipple? He'll find it icky, and then you can offer him the sippy cup right at that moment to wash away the icky taste. It's a pretty gentle form of aversion training. Just try to be as surprised as he is and very sympathetic. Lots of hugs and tell him how big he's getting and how proud you are of him. DON'T let him see you do the actual 'flavouring'. Kids are smart and put things together much more than we think.
Because I was so late in weaning him, I did let him have a bottle with water in it at night time for about 6-8 months later. He doesn't even ask for it anymore. Good luck. and a big hug for you!