I am a mature student (30s, married, 4 dc etc) and have been through the process of DSA and assessments etc. This may be long, sorry 
I am Bipolar, which I had to provide medical evidence for, this was just a signed letter from my GP. I applied after I had started uni, the first year I struggled through not realising help was on offer, but second year (which I have just finished - yay!) I have been quite ill, mentally, and accessed student services when I started the year.
They filled in the paper work for me, they arranged an appointment with the assessors, and they put in place things they assumed I would be awarded the DSA for before I had even gone to assessment. For me the options on site at uni were mentors, scribes, note takers, and work facilitators, this was all in addition to the usual academic support everyone can access like writing workshops and the like.
I chose a Mentor, though I could have chose all the options had I wished. I see her for an hour each week and we plan workloads and assessment timetables (often she gets hand in dates moved for me to create an even spread across the year - worth its weight in gold!) She also acts an advocate for me contacting individual tutors in the modules I take and informing them of my condition and any reasonable adjustments they should make - sending me the power point in advance, allow me to use recording devices, and things like that.
The assessment was not what I expected at all - I honestly thought I would have to 'prove' how mental I was or something. Its not like that - they are there to make sure you get the absolute maximum you need. They gave me programs for the computer for mind mapping, speech to text, text to speech, a live scribe pen that digitally records the writing I do, and can record sound, so the lecture, at the same time, a Dictaphone for recording in large lectures. They supplied a laptop as mine wasn't good enough to support the programs they gave me. They even sent batteries and battery chargers and extension leads in my pack! It was like Christmas!
They ran through the 'people' options, like uni already had, and were happy to fund all the services if I wished to have them, but I stuck with just the mentor as I didn't want to many extra appointments with other people, busy enough as it is!
They asked if I needed any help travelling, or if a free parking permit would make things easier, (these didn't really apply to me, but the offer was very much there). They also give me £300 a year towards books/paper/printing.
In the report they send to the uni it specifies the reasonable adjustments they should make to even the playing field for someone like me. So things like unquestioned extensions and moving assessment due dates to offer an equal year, extra time in exams, (actually they have given me take home open book exams) having access to power points and before the lectures along with any hand outs that are to be given, being allowed to use laptop/recording devices (some tutors are funny about this until they have the nod from student services saying its required.)
At the centre I went to they seemed to match the assessor with the needs of the student, so I had one experienced with mental health issues, my friend had one who solely saw dyslexic students. I don't know if that works the same at all centres though.
I hadn't known what to expect, but at least I had already experienced uni environment to be able to properly answer the Qs, I imagine it would be harder to do that without the experience. That said, my assessor was lovely and very much 'on my side' and ran through many scenarios and options that are usually offered and how they work and then worked out a 'best fit' for me.
Sorry that's so long, it has been a really positive experience for me, I hope your DS accesses the services on offer as it can make such a huge difference. But, even if he doesn't in first year, he can always go back for a re-assessment if he wants to.