Hi OP - lanyards are fast becoming meaningless in all but a few specific situations, like air travel or perhaps negotiating a long train journey. One reason being, that people like to identify into a 'tribe' and in order for others to recognise this, they need an identifier. I do get that, actually - but the sunflower lanyard used to be intended as a universal signifier that this person had a perhaps less visible disability which meant they might need extra space/processing time/extra understanding/assistance/etc and might not be able to tell officials or other members of the public.
Now, when so many people wear them (though I've noticed a bit of a fall-off in the last couple of years) they're just part of the scenery.
I would encourage your DS not to think of his autism as being part of a 'special club'. It's part of his neurological make-up and probably informs his personality, but it isn't his whole self and he probably shouldn't be encouraged to think that it's in some way special or exclusive. It just is, like his hair colour. (Yes, I am being disingenuous. Of COURSE it's a bit more than that, and of course many autists view their autism as absolutely integral to themselves, but if your son's reasonably functioning and able to initiate conversations with people then I think he needs to be viewed not just through the lens of autism.)
Look, lanyards are there to perform a function - in specific settings, to allow others, mostly officials who can help, to be aware of a condition (not always autism!) that might need assistance or extra understanding. This obviously depends on the level of personal understanding, tolerance and training any individual has, so don't expect miracles there. Lanyards are NOT meant as a badge of honour or identity. I bloody well wish they'd stop being used as such because it makes them so ubiquitous that they've all but lost their meaning and use.
Anyway yes, I believe Tesco still give them out free. And airports. And probably most large city train stations. Don't waste your money on one that someone is selling and that looks a bit different - what on earth would be the point in a lanyard aimed at quickly indicating to others a less visible disability, if it looks different?? 
Oh and you don't need a card at the end. What would it say??