When you offer the sippy, do you do it in the highchair? Sometimes that can help, or at least not giving it in your normal bfing place. I also gave dd a small snack, a couple biscuits or some fruit, for her snacks that replaced nursings. Avoid that nursing spot, keep him busy with other activities.
If he is so reluctant, you could try waiting a month or two and trying again. Not sure what your motivating factor is again, I apologize. I found that my 14 month old is a much more independent 'baby' than he was at 12 months old. They change so much!
(He teethed all four of his molars almost overnight at 13 months - have you checked that yet?)
