Before we take this question to a lawyer, it might be useful to ask it here and see if anyone has gone through this already.

My wife and I are residents in Spain. She is an EU citizen and we are both US citizens. Our child was born in January, and as we understand it, after one year he *should* be eligible for Spanish citizenship. We have already obtained his US and Bulgarian passports, and the question is, would we have to renounce these, and to whom, and what would the repercussions be? Obviously, for the moment his status as an EU citizen affords him the same rights as Spanish citizenship, regarding access to health care, scholarships, etc., but we don’t know what the future may hold, and if he has the right to it, he should not be denied it.

andy-bob nolen,
your child is automatically bulgarian and usa citizen.
but you have to choose which passport he is using as a resident in spain.
as bulgarian he is an eu citizen.
so if you apply for spanish citizenship in his behalf, you renounce for him the citizenship of the passport he uses as resident.
you will have to get a certificado de registro for him to prove the one year residency.
now the renunciation is carried out by his (in this case, your) declaration in front of the judge when he (in this case, you, on his behalf) swears allegiance to the king and constitution.
his spanish birth certificate will then be amended with his acquisition of the citizenship and his mom’s last name is added as segundo apellido, you can get him a passport and dni after that.
bulgaria has laws on how to lose citizenship, and may consider him bulgarian even though you went thru the renunciation of the bulgarian citizenship because this may not be enough action in order to lose bulgarian citizenship.