I have two sons - a 2.5 year old and one that just turned 17 months. I breastfed both. My oldest was easy to wean. I worked part-time in the evenings so he was used to getting his milk out of a bottle or sippy cup, so when I found out I was pregnant again when he was 9 months old, I had no problems transitioning him to formula.
Now my 2nd son is a different story. I quit work to stay home with both boys permanently when he was born. Both boys loved to nurse, but my 2nd son, I never pushed the bottle or sippy cup much early, because I didn't have to. He would drink from a bottle or sippy cup daily, but usually only about 5 oz. and then he was done with it and wanted the breast again. My goal was only to nurse to 12 months, since I was forced to stop earlier with my first son. I just wanted to make it to the point where they were allowed to drink whole milk so that I wouldn't have to buy formula. Plus I enjoyed the bonding nursing gave me w/ my boys.
Two weeks before my youngest son's 1st birthday, I started the weaning process. I had worked hard in the weeks prior to that pumping and freezing the milk so that I'd have milk to start weening. I started by skipping mid morning and mid afternoon feedings w/ the breast so that my milk productions would start decreasing and I offered him thawed milk in a cup. He showed no interest in the cup or bottle. He wanted it from the breast, which I wasn't caving in. He was stubborn and would only take a few sips from the cup and would just hold out until I did nurse him again. After a few more days, I stopped nursing in the middle of the day and just nursed when we woke up in the morning and just before I put him to bed at night. He still wouldn't drink much from the cup at all, and I wouldn't cave in and nurse him. He did eat baby foods and some small table foods, so I made sure to pick baby foods with water like the fruit and veggies. So I knew he wasn't starving, but yet I knew he wasn't getting as much milk as he was used to. I then cut out the evening nursing only nursing 1 time each day the last 3 days of nursing. He wouldn't drink much for me at all, pushing his cup away wanting the breast, I wasn't going to cave. I wanted to start taking normal birth control pills too. The pediatrician told me that he wouldn't starve himself, so to just keep doing what I was doing. After 3 days of nursing one time a day, I was done. I would not let him have the breast at all and I continually offered him milk in a cup - now at this point, it was whole milk. For a good 24-36 hours, he refused to drink the milk. He was very fussy during that time, but I was stubborn enough to not cave in, even though it was very hard for me, as I did not like seeing him upset. I continued to give him his regular meals 3 times a day; baby foods like fruit and veggies so that he was still getting water in his food. As long as a baby makes 4 wet diapers a day, then you know he isn't dehydrated. I continued to offer him milk and like my pediatrician always told me, a baby will not starve himself. If the baby is hungry, he will eat. 36 hours after I stopped nursing altogether, he finally accepted the cup of whole milk and began to drink it and I haven't had any problems since then. It was the hardest thing for me to do and it wasn't easy. He was cranky more often during the 2 weeks that I started the whole weaning process, but I knew that if I caved in and gave him what he wanted, that I'd just be prolonging the whole process.
The point to the whole story is, make a plan and then stick to it. Continue to offer your son milk, etc. in a cup without nursing. It won't be easy, but my pediatrician has told me time and time again, that your baby won't starve itself. When it's hungry or thirsty enough, he will eat/drink.