What if our biological child is changing his mind about adoption?

New to the forum, looking for some input... We have one biological son (7 1/2yrs old), and have been going through "the process" for coming up on three years now. We decided against having more children, but began to pursue adoption to give a home to a child in need, and to give our son a sibling. Initially our son was fully in favor, and we talked about what his brother might be like. Lately, however, our son has began to waver on that opinion, and now suggests that he'd prefer that our family stay just the three of us. We're torn between knowingly proceeding with something that our son might resist (possibly resent), and just assuming that we know what's best for our family, hoping that our biological son will someday realize that it all worked out well. Does anyone have an experience with this? Any feedback is appreciated, thanks! dan m ///