You should talk to a licensed social worker. We used Oasis,866-579-5578, so you might give them a call. If, in the future, you were to become sole parent you would want/need something stating you are the parent. You'd need this for medical reasons, for school registration, etc. At that point you would perhaps need to have a BC with you listed as parent.
Ask the social worker if you could show the adoption papers for all these things instead of having to use a new BC. Also, ask if the new BC necessarily makes the old one void, or could he just have 2 legal BCs that are both valid?
Other questions to consider... if you were sole parent, would your son necessarily want to use adoption papers for all those things and thus have people always asking questions about his being adopted? He may not want that revealed to everyone. Also, if he were in the hospital and you had to make medical decisions, would they readily accept the adoption papers, or would it just be easier to show the BC?
Just some things to consider. Our situation is different since our son is adopted from another country, but I do know that when people encounter those adoption papers, they are often reluctant to accept it as "proof" simply because they are unfamiliar to them. Our son's original BC isn't in English, so that poses problems as well. You might find that it would be easier, from a legal standpoint, to just have that BC with your name on it.
I'd get numerous legal copies of the original BC for him to have and keep before doing the adoption. Then, you two can have a talk about why there is a need for another BC and that it doesn't take away from his mother's place in his life, that he has 2 mothers.
Call Oasis or another SW... I bet you can get most of your questions answered on the phone. You are smart to be considering this and taking care of the legal things. Even if nothing happens to your husband, you need to be able to make legal and medical decisions if your husband was away or something. Good Luck!
PS - Marie, I know it sounds weird, but you do indeed get a birth certificate. Our new one lists me and my husband as parents, but still lists his foreign town and country as place of birth. Without this, you DO/WILL have people questioning if you are the parent and whether you can make decisions on his behalf.
Even though your husband writing a will is a good idea... if you're thinking of all the "what-ifs"... What if you husband and son were in an accident and your husband was not conscious? Would the hospital still let you make decisions about your son? You'd encounter the same issue if your husband was traveling and your son needed surgery. Would they accept your decision or need a "family" member? Not to be morbid, just trying to cover all the possible scenarios.
Having you named as legal guardian sounds like a good idea. Maybe that would cover all the legal and medical "what-ifs" and also make you able to make decisions regarding your son while your husband is alive and well! You are smart to deal with this now and not when there is a crisis. :)