Hi all,
Apologies in advance since this is pretty long but I could really do with some advice so thank you for reading!!
I posted with some concerns about this a year ago. At the time DS1 was just turning 2 and, compared with other kids of around his age, he was strikingly non-verbal. Some of his tantrums were pretty apocalyptic and his social skills weren't exactly non-existent but they were certainly under-developed. At the time, he was going to a childminder with a small number of other kids around his own age 2 days a week, he was with my MIL 1 day a week and with me the rest of the week and we were all keeping a close eye on him.
Advice on here suggested I should phone the health visitor which I did and she referred him to audiology and for speech & language assessment. He has had two audiology appointments over the last 12 months which have established DS's hearing is not the issue, though he is definitely over-sensitive to certain sounds. Not all loud noises set him off - for example sirens or lawnmowers etc are fine but hairdryers, hand dryers, vacuum cleaners and a hand drill all set him off, to the point where having to use public toilets is an absolute nightmare. Even when the hand dryer is not being used, the very sight of one sends him screaming and running for the door 
Speech and language assessment has been a slow process over the last 12 months. On the one hand, DS's verbal skills have developed substantially and he talks a lot better and with more confidence and a much wider vocabulary now. He is still hesitant about talking to strangers and can be very shy, but largely speaking he can answer simple questions, follow simple instructions. He is a really bright kid, in that he knows all his numbers up to 20, all his letters, can spell out / read simple words (and he only turned 3 on a couple of days ago), shapes, colours, he asks lots of questions about how things work etc.
On the other hand, some of his behaviours (like the over sensitivity to sound mentioned above), along with being fairly slow to master toilet training (he's getting there...but slowly), a complete inability to share or take turns with other kids, huge melt downs and tantrums when we are out of the house (screaming, tears, lying on the ground, shouting), a real dislike of anyone apart from our closest family members visiting the house, are very concerning. Around the house, he's not too bad, a bit of a handful but relatively easy going. Outside of the house (or if we have any visitors round, even for a short time), whether we're at the supermarket, the park, anywhere really, he is completely unpredictable and you never know whether you are going to get a typical toddler or really terrible behaviour.
Things have been intensified by the arrival of DS2 a couple of weeks ago, DS1 is pretty keen on his new baby brother, but his tantrums and other negative behaviours have increased substantially and he is supposed to be starting playgroup in a week's time and I have no idea how on earth he will manage it.
We are starting to feel a bit let down by the system. It's been 12 months since health visitor first sent DS for referrals. Audiology took 11 months to complete their assessments, and the consultant told me he definitely felt that DS should be referred for ASD investigations and he would write so in his notes, and we should pass that on to speech and language. He had another appt with them a few weeks ago and the therapist he has been seeing seemed inclined to agree, despite his obvious progress in using language, that he is probably somewhere on the ASD spectrum, albeit fairly highly functioning. However she has now left her post and we are supposed to have another appt in October with her replacement to discuss the possibility of ASD investigation referrals and do a questionnaire. It all feels very slow and meanwhile, me and DH are finding DS1's behaviour increasingly challenging to cope with, especially with a new baby to look after. I am at the point where I am wondering if I need to consider not returning to work in the new year when my maternity leave comes to an end, because I don't feel that we can leave MIL to cope with DS1 and DS2 when DS1's behaviour is so tough to manage. But I find the prospect of being at home with both of them daunting, since DS1 is such a handful and we have had no real help or support at all. I also feel for him because he is very sensitive and affectionate and he spends a lot of his time upset, stressed out, angry and frustrated, which can't be hugely beneficial to him.
So, where to next? What do ASD referrals usually involve? How long do they take? What can we expect? And does anyone have any recommendations of books we can read, websites, anything at all to keep us going in the meantime...? Thank you! Sorry again for such a long post 