American citizenship by descent

My husband is a British citizen (age 55), he has recently found his biological father (age 76) who is an American citizen. At the moment we are in the process of changing the father's name on the birth certificate. It has to be done through the court. Once this is completed, will this amended British birth certificate be enough to successfully apply for the American citizenship by descent? Can it be done without involving expensive lawyers?

US nationality law by birth outside the US is quite complex and depends on the period of time that your husband was born.

Have a look at the Wikipedia page on US citizenship through birth abroad to US citizens to get a broad idea of the complexities involved.

Thank you. It doesn't look good. His parents weren't married at the time of his birth. His father was serving in US Air force and was based in the UK. I was hoping there were special exceptions for servicemen, but probably there was nothing like that.