Tell him not to worry
. I had a DSA assessment for myself as I was diagnosed with dyslexia (only when I had finally reached university, it was ignored throughout my school life). I also have a DS with HFA and the assessment and application process for DSA was miles easier than what I had to deal with for his statementing. There is no fighting over funding at all, they are really keen to get students to be fully supported and if your EP writes that you need a certain piece of equipment/software, there is no dispute over it, it's as if they have their chequebooks out ready for you!
Most of the information they had about my condition was through an EP report, where I'd had cognitive and handwriting tests done. Has your DS got something like this, or reports from other professionals stating what he needs? Then at the DSA assessment itself, they were mostly asking questions about my lifestyle, study schedules etc to see what kinds of software/equipment would be most suitable for me. It seemed that they had read through the report, picked out the recommendations and just added them to a purchasing list. They also offered things like handwriting grips, a fund for limited taxi costs, a support worker, photocopying costs.
I'm not clear whether the assessment you mention is more like the EP assessment I had (where they assess your abilities and needs and work out what support you'd need) or the Needs Assessment (where they decide what to give you - more an administrative thing). He would have to respond/participate for the EP assessment, but for me it was mostly writing down answers or one word answers for cognitive tests. I am not sure if he'd need a recent EP assessment if he has a history of professionals being involved though, did he submit lots of reports when he made the DSA application?
I didn't bring anyone to my assessment but I don't see why they wouldn't allow you to observe/support him if he agreed/requested it, even if he's 18. The only issue might be with the EP test in case they're worried you might influence his answers, but I have had to sit in on EP tests for DS because he would actually refuse to enter the room if I didn't go with him.
Well done on your DS for getting to college though - it's a real achievement with all the difficulties our boys have
.