At 15 months, I'd skip the bottle altogether, and start working with him on taking a sippy cup. Kids, even babies, want to be like the adults around them. Since a sippy cup looks more like what we use, they will often go to a sippy cup when they are not at all interested in a bottle. Then, gradually cut out the times you nurse. Try not to make a huge issue out of it, if he feels you are tense, he won't know why, and it is more likely to make him cling to breastfeeding longer.
Some people also have a lot of success with using a special blanket or stuffed animal while they nurse for a little while and then switching over to using it at the same time as the cup. The baby associates it with the comfort they get and it makes the transition easier. It works well for some, not so well for others, and it does take some time though.