Hi. Get tested for lymes disease at a proper lab. UK has poor testing for lymes and it can be easily missed.
Look at armin labs in germany. Its around 350 to send blood from UK to germa y.
Its still worth asking NHS to test him for it. Although not many drs believe in it and have no knowledge.
I've been thinking about your post for the past few days, and up until now I have not answered it, because I do not have an answer for you, but I have decided just to share some of my thoughts, FWIW. First of all, what has happened to your brother is obviously pretty devastating, and I'm so sorry for the situation. From your comments, the CT and MRI scans evidently have not shown any kind of tumor, blood clot, or other structural abnormality, aside from the fact that "his brain has shrunk slightly since his last scan a month ago." Any measurable shrinkage (the medical term is atrophy) in only one month's time is remarkable, and especially so at age 26. Middle-aged and elderly people often have some degree of brain atrophy that can be considered normal, but not 26 year-olds. I'm not a doctor, but I work in the psychology field, and a finding of measurable atrophy in one month's time -- in a 26 year-old -- really jumps out at me. Rapidly progressing brain atrophy in a 26 year-old is, I think, a big deal.
The other thing that occurs to me is that the doctors HAVE to have some idea of what the diagnostic possibilities are. I think they probably are playing it close to the vest, possibly so that they don't have to explain a bunch of diagnoses to the family that don't turn out, in the final analysis, to be your brother's diagnosis. But they have to be working through a list of possibilities and using the test results to eliminate things off the list, as they go. Your brother has had a lot of tests already, so I think it is reasonable for the family to ask the doctors, "what are the possible causes of my brother's problems, at this point? What possibilities are left, that you have not yet eliminated?" If you do decide to press the doctors for answers, I don't have any idea what they will say, but I know there does come a time when anything is better than not knowing. I will be hoping for the best for your brother.