It's been a few years since we've been, but this is how it works I think...
The paediatrician's letter should be enough to get the pass, but bring the DLA award letter too. They are basically looking for something from a medical or educational professional to confirm whether he will have difficulty queuing, and a confirmed ASD diagnosis will do it.
You may be asked questions by the person issuing the pass - they will probably be reading these off a script, so don't be surprised if they seem a bit clueless. Any problems getting the pass, then politely (but firmly) ask to speak to a manager.
That should be enough to sort you out for the entire season -also, when we went, one carer used to get in free, and I'd be surprised if they've changed that.
The ride access pass isn't as comprehensive as it used it be though. Merlin's customer feedback told them that NT folk think that these disabled passes are "unfair" (yes, you read that right, the poor lambs), so Legoland have restricted things a bit.
With a ride access pass, you now get about 10 opportunities during the day to skip the queue and get on the ride via the exit. It applies to your DS, and up to 3 other people in your group. DS will be given a red stamp on his hand - it washes off easily, but you can get another stamp without any bother.
One important thing to bear in mind is that you normally have to wait a while (about an hour, usually), before you can use this pass on the same ride. So if there's one ride that your DS really, really likes, then he won't be able to go on it again and again without queuing up.
Hope this helps!