Getting help with speech and language as an adult

My brother (20) has what could be global development delay or speech delay, we aren't really sure as getting any diagnoses and proper support catered to him has been difficult. He's able to hold conversation and read fine most of the time, but has trouble with both taking in and expressing more "complicated" ideas/instructions and using grammar, which has given him trouble when dealing with relationships, emotions and his schoolwork. He has gone to specialist schools in the past and has gotten SALT and other support through his time in school, and through his hard work has attended a mainstream college the past few years. He's been having trouble with passing his English and Math GCSEs and it's really brought him down because it meant he wasn't able to continue to the next level of his chosen course.

We're not really sure what to do as he isn't really happy with how things are now, but it feels like schools and the council are kind of just bouncing him around instead of trying to help us find support that's right for him. When we look at things in terms of issues with speech and language, most are directed towards children and options for adults seem sparse. We know that GCSEs aren't everything, but it would be very beneficial for him to be able to express himself more and feel more independent and capable to pursue the career he wants to. I was wondering if there were any services or options for adults to get help with cognitive and expressive language in London? He hasn't gotten a professional diagnosis for autism but we're not sure how worth it is pursuing as language and speech is the biggest issue for him, and he also gets a bit uncomfortable and saddened whenever I've talked to him about it. When we talked about it with one of his high school teachers at one of the specialist schools he attended, they also said it wasn't necessary.

I'm not sure if I worded this too well but our situations been kind of complicated and I'm not really sure where we can turn to now.