K.F.
Don't worry about weaning him. If you're not around he will use the sippy cup if you are around and you are busy tell him he is going to have to wait a minute and if he is really thirsty he'll go for the cup.
I intentionally weaned my daughter because she was still eating much too close to a two hour schedule by the time she was about a year but with my other two kids, and my youngest nursed until he was 2, they pretty much tapered off without my having to be too gung ho on eliminating the feedings.
Your other option is eliminating another feeding instead of the one you are trying to get rid of. That time of day they are usually wired and ready to go but around the afternoon they don't have as much energy to fight you.
If you have tried several times, he just isn't ready yet and continuing to try to do it is really counterproductive. If you want more freedom, leave the baby with a friend, relative, whatever, because if you're not there, the issue isn't there and he will use the sippy cup and before long he will be too busy to worry about it and then the weaning will be easy.