P.M.
It sounds like the two of you would benefit greatly by seeing a counselor. There's much more going on here than naming this particular child.
My aunt changed her son's name when he was a toddler...odd...but he/we all got over it. The guy who cuts my hair went by his middle name until he was 30 or so...then decided to use his first name. Takes a bit of time to get used to a different name @ 30...but it worked. A friend of my daughter's is called by her middle name at school (the name her mom liked) and by her first name at home (the name her dad liked). You can call him whatever you want. It doesn't matter what's on his certificate.