One question, if my husband were to adopt my son (adult adoptions are allowed in our state by stepparents) then would my husband be able to apply for a British passport for him?
If a child under the age of 18 is adopted by a British citizen, then they do have the right to a UK passport (I know this to be true because my aunt is a dual US/UK citizen who adopted 3 children in the US and they all have British passports).
However, I can only find information on the UKBA website about adopted children under 18, not about adult adoptions, so I'm not sure if you would be able to do it or not.
Does your son want to become a UK citizen though? You mentioned in your other post that he is 24 and doesn't see himself living outside the US.
Surely as he is an adult, if you want to move to the UK, then that should be your decision and it shouldn't be based around or affected by your son. He's old enough to be independent, to make his own life decisions, and to deal with it himself if you do decide to move. I know you say you are close to him, but at the same time, he's not a kid anymore and you shouldn't have to base your life decisions around him.
My parents have 3 kids - we're aged 29, 27 and 22 now and I'd say we're pretty close (at one point a couple of years ago, we were all living back at home again), but that hasn't stopped any of us from moving to different countries and living thousands of miles apart from each other.
Recently my parents have been seriously considering leaving the UK and moving to Austria. If they do, it will be entirely their own decision and us kids support whatever they wish to do - we have our own lives now, so what our parents choose to do isn't really anything to do with us anymore.
Having said that though, my 22-year-old brother won't support a move to Austria, but that's because he lives in Austria himself and he doesn't want our parents living there too... as he says, 'I'd rather you didn't come here, because moving to Austria is my thing, not yours'

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My other brother won't care either way because he lives in China and will only see the family once in the next 2 years anyway

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I'm the only one of us living in the UK still, but I have also lived in the US on my own twice (at age 20 and 24 - if I had stayed in the US for 5 years as I had planned, my parents would have had 3 kids living on 3 different continents by now

). I will also be moving to the Falklands in the summer for 5 months and then to Ascension Island for 3 months in 2014 (all for work), where there will be almost no internet access and phone calls will be extortionately expensive... so it doesn't really bother me as I'm not really going to be able to see or contact them much in the next two years anyway.