It is not a question that anyone can answer except yourself and your family. Adopting is a lovely gift to a child without parents but can also be a tough thing for another child to handle depending on how it is dealt with.
Some parents chose not to tell there biological child while others will. I personally think it is better to tell them from the start. It is entirely up to you. If you sit your 6 year old son down and try in a way he will understand, explain to him what mommy went through, what it means. Explain to him your idea, of adopting another child, someone for him to play with...etc. You will need to let him know what will change and everything. Make sure he fully understands and see how he reacts.
It may take some getting use. I hope this helps.