My husband has been a part of our son's life from age 3.  He's 10 now.  My son's biological father (BF) was in and out of his life for about 7 years and in that time remarried, had 2 other sons and had asked my husband to adopt my son saying it would be "best for him".  My husband would have jumped at the opportunity the first time when our son was 6 but after much discussion, we weren't sure it was the right thing to do.  When I say that, I mean we were worried about comments later on in life like, "Why did you take me away from my real dad? and It's your fault I don't see him anymore!" 

At age 9 when his biological father wanted to give him up again, we jumped at the opportunity.  This frees him up from the arguing that always occured between his BF and new wife and his constant adjustment and readjustments when his BF would pop in and out of his life!  Our son is happy as ever and is proud to have our last name.  We left his name exactly the way it was and just added our last name to the end as to not take away his identity.  Our son is also very close to his BF's grandparents, aunt and uncle. 

I dread the possible questions later on in life though...