Regardless of his sexuality and who he is going to, as mother you are in your right to not like the idea he is going so far from home. Its something that most mothers will have an issue with.
A little truth: As a gay man myself, if I had children I would secretly pray they were straight. Not because I have issues with homosexuality, but because as a gay man in a predominately straight world I know all too well the terrible things that LGBT receive at the hands of haters.
I think no parent wants their child to be hurt... And yes, being LGBT opens one up for a lot of hurt. I'm sorry that that is the way it is with our world, however things are changing and tolerance is growing.
Being gay isn't a problem, it doesn't hurt in and of itself. Yes we get treated badly, but being LGBT isn't painful at all. No more than being straight is painful.
Men tend to struggle more with the whole 'you're gay' issue than women. Its a combination of assumptions that gay=emasculation (males are pretty fond of their penises) and most men are raised to be out of touch with many of their emotions.
So its usually a longer road to acceptance for a straight father or straight brother to reach the point where they are ok with it in a person they love.
Yes there is a risk your husband may never come to fully accept the gay son, however if your husband has any amount of love for his son, he will come to tolerate it and accept it to a certain level.
Yes, your son is taking risks in his move - many of them. However those risks are offset with a very real chance he may find real happiness, love and all of the things he needs to be happy in life.
There is an organization for Family and Friends of LGBT, there are local chapters in the UK:
http://www.pflag.co.uk/
Perhaps you should seek out a local chapter and get some real face to face friends with other parents who have 'been there, done that' and have real immediate support.
I think you will be amazed at how similar their stories are to yours, and the huge heaps of advice as to how to deal with lots of stuff.
I know there are a few members here on GaySpeak who are parents of LGBT children. I do hope you and they can have serious conversations.
I would say you are doing real great, you are doing the right things and seem to have a serious desire to right by your son. You are a good mother.