Single mom of 6 yo boy looking to adopt

I am divorced single mom of a 6 year old boy looking to adopt a boy 4 to 6 years old (younger than my son so he can be the big brother). I have spoken to my son about the adoption and he seems to be thrilled about having a brother. However, I'm concerned about how he will react once another child is in our home. He has been my only child for so long (he's my biological child) and he's very close to me. Adopting is something I always wanted to do. I have discussed it with my ex-spouse, but he was not sure about it. But now that I am single and have a child, as I see him getting older, I don't want him growing as an only child. However, I want to make sure that bringing another child into our family will be the right decision, especially because of the closeness in age, which is what I'm looking for. Are these feelings normal? How do I explain how things will change to my child without having him feel replaced? So far I have told him, that having a brother will mean he will have to share with him toys mom and time, and he says that's ok. Somewhere inside I feel that it will just be fine and that they will have a blast together... but would like to hear from your experiences.