adoption of adult former foster son

Our former foster son, now 19 years old was with us for 9 months before aging out. He is now excelling in college and would like to be adopted. We have four biological sons whose ages are 31 (married with 2 year-old child), 29, 25 (married and expecting), and 22. All of these are out of our home. The older two and a wife are jealous of my (mom) relationship with new son, with whom I am very close. The younger two and a wife are very open to the idea. There is no question in anyone's mind that we are his "forever family", but legal adoption seems to be a stumbling block to some. Money and inheritance is not the issue as we can wait until after college which is paid for by scholarships. My husband wants total agreement by all the kids and so is holding back as well, saying "give it time". I understand what legal adoption would mean to this son's heart, like the commitment of marriage rather than just living together. I am impatient! Has anyone had a similar experience?