I'm currently at university and have been recieving DSA for the last 3 years due to an anxiety disorder, but next year I'll have to add on that I have diabetes.
When you have the needs assessment, you can get some great stuff that might help out! One of the best things I got was a voice recorder to use in my lectures, this could benefit your son as if he goes high or a little low in class his concentration may be disrupted.
It's also possible to get laptop warantee's and other equiptment such as printers... but these you have to buy yourself and then claim back through DSA. I don't see why these would be necessary with diabetes though.
I also had the option of recieving money back on taxi fares, if I were feeling anxious one day I could get a taxi and the money would be refunded - as long as I got a receipt from the driver. This could be good for your son also, the low days we definitely don't want to walk to class and the high days we're too tired to!
He may also be able to recieve counselling, which I personally would take and see how it goes... being at university may be stressful and it would be good to have some support for him in place, just incase. He can always opt out at a later date, but once he's refused it's very difficult to get the support.
I know it's late now, but you should still be able to apply, just phone up student finance and ask. Also, if you've applied and had the assessment already, there's always next year to apply and get further assistance if he'd like anything I've suggested.
I'd say go for it, because it is listed as a disability on the student finance website and there's some great things that he can rely on even if he doesn't use them, it's great to have something to fall back on. Where is he going to university? If it's anywhere near mine, I'll keep an eye out!