Hi. I just had a baby on July 30th so ours are pretty close in age. ;) I would agree with the first opinion. I'll bet it's hard for your Mom to see you as a mother (I mean that it's hard for her to change her perspective). She sees herself as a mother and you as her child, so when a baby comes in the picture, she acts as the "mother."
In the beginning, there is some transition time and getting to know your own baby. That means that there are some sleepless nights, cranky baby times, and frustrating moments. Your Mom was probably trying to "rescue" you from these things because she loves you so much.
I would say something to my Mom like, "Mom, you have helped me SO much with _____. I know how much you love him and can tell that he adores you too. I am feeling like my son is not as attached to me as I'd like him to be. I want him to know that he can trust me, as his Mom, to take care of his needs. That means, that I have to be the one to respond to him most of the time."
Then, I would make sure to not treat your Mom as a babysitter for a while. If you have to run to the store, take your baby along with you. If you are meeting a friend for lunch, take your baby with you. Show your Mom that you are capable of mothering and show your son that you are there for him.
Things will DEFINITELY improve with time - especially since you desire a stronger bond with your baby. You are blessed to have a mother who is willing to help you. Your son needs as many loving people in his life as he can get and it will be wonderful for him to be securely attached to you AND Grandma as well. He will feel like a million bucks as he's growing up.