Don't create that habit of him needed someone to always rock him to sleep. You have to give him the confidence that he can put himself to sleep when he is tired. That way if someone babysits him, it'll be easier for someone else to put him down.
With me second child I learned to establish a strick nap time and now he is 23 months and he loves to go to sleep when he is tired. There are sometimes i will notice that my son is really quiet, and i'll check in his room and he will be asleep in his bed.
When it's nap time, tell him "ok hunny, it's time for you to take a nap, your getting tired", put him in his bed, give him kisses, tell him you love him and leave. He most likely will cry the first couple of days untill he realizes this is nap time and mommy is not going to get me. Allow him to cry for 15 minutes , check on him and tell him again, it's nap time you need to go night night, and that you are going to be alright and mommy loves you. Then allow him to cry.It will get easier with much patience. Try keeping yourself busy, like doing the dishes, if you sit there and listen to your child cry, you will want to give in. Remember, crying is normal ecspecially if they are tired.
Also try and see if you can get him a blanky or a stuffed animal that he likes to give him the comfort he is looking for. My son was breast fed also and I gave him a blanket and always laid him down with it and he grew to love it, which made up for the comfort he looked for in breastfeeding.
You can get through this, just stay consistant and be strong!