Hi, my DS was diagnosed aged 6, but I remember mentioning it to the HV at his 13 month check-up.
Now DS is 8 and thriving, but we're requesting an assessment for DD2.
Because I knew there was something slightly different about DS, I was reading about autism etc from an early age. I think the things that really helped were: believing him when he says he's scared or upset, even when these seemed like massive overreactions; being as explicit as possible about anything like instructions or affection (I tell him I love him all day every day!); trying to make sure he is supported through any transitions (this can be as simple as telling him we're turning the tv off at the end of this programme, or as big a deal as making sure he gets extra time in next year's classroom at the end of the summer term).
Basically, just make sure his individual emotional needs are met - you can't expect a child with ASD to be as resilient as most children to start with, but with this very supportive start, it can make life much less difficult for them.
He is now very confident most of the time and manages absolutely fine in a mainstream but small and supportive school.
Being autistic isn't necessarily a bad thing - many of the problems come from being expected to be normal and treated as deficient or naughty when that doesn't work. So as the parent, if you spend the early years reinforcing to the child that they are safe and loved by being sensitive to their needs, that will make a huge difference.
Have a look at the Transforming Autism website. It's a bit full-on, but the idea that the main thing autistic children need is to be contained emotionally is very powerful.