Hello Calamity - just seen your post and felt I had to reply. I have Usher type 2 - I'm 41 and was diagnosed at age 13, so similar to your DS. I understand how distressed you must feel right now - I know my mother was devastated at the time of my diagnosis.
I am in the middle of working right now so can't write too much, but I wanted to say, don't feel like this is the end of the world. Despite the Usher's, I have a very satisfying life (good degree, interesting work, married, three kids etc. etc.) There are lots of difficulties associated with Ushers but they are not insurmountable.
Have you contacted Sense or BRPS? I would definitely get in touch with Sense (you can find their website online) - they are a charity specifically for people with dual sensory loss, and I believe they have some good counselling / information available.
There is a large amount of research going on at the moment, and I personally think that there is a good chance that some form of treatment will be available at some point for your son.
reallytired - I have not heard of people with Usher2 completely losing their hearing. My hearing loss has been stable my whole life, and my understanding from specialists is that is normal. People with Usher type 1 have a more profound hearing loss from birth, but again that is normally stable.
I don't have personal experience with language or information-processing disorders which I completely understand complicates things for you and your son. But Sense are likely to have experience with this.
My personal advice, for what it is worth, is to not hide things from your son, but not to go overboard with explanations either. I knew that I might/would go blind from the time of my diagnosis (the doctor told my mother within my earshot) but it didn't feel particularly meaningful to me then. And, really, with Usher 2, the timescale of the progressive sight loss is so long that there is no point in getting too worried about that straight away.
Please do get in touch with Sense, if you haven't already. They can be enormously helpful.
If there's anything else you want to ask about Usher 2, then fire away, I will be happy to tell you anything from my own experience.