I would be cautious about adopting a child very close in age to your present child. What will you do if they don't get along? If they are competing, will you honestly be able to say you love them equally and do what's best for both of them, even if it is not what your bio son wants? If you adopt an infant or toddler, your two children will not be directly competing, and that will make it easier for you to establish family bonds.
The way you talk about adoption makes me wonder if you have thought it through. For example, your son will not be adopting anybody. You and your spouse will be the parents of this child, your son will be his/her brother. Is your son hoping for a live-in friend? That would be nice, but it is not adoption.
Finally, if you will not consider yourself this child's "real parents," you should not adopt. Adoption means becoming a child's parents, in every way--legally, emotionally, day in and night out--except biologically. A 6-year-old who is in foster care will be hoping and waiting for Forever parents. You will need to establish a loving bond with that child, and you will need to establish order in your household, but you will not need to debate with the child about who are his real parents.
If the reason you want to adopt is because you want to expand your forever family, to bring more love into your home and to help a child grow into a competant, thoughtful, loving adult who will contribute to society, then by all means, go forward with your plans. But please, think it through first and be sure of what you are doing. Because adoption also means one more person in your family to worry about, argue with, and take up closet space.
Best wishes with your decision,
Xanny