Group speech therapy was wonderful for us as well as our son having access to an amazing nursery teacher who took him and two or three others in the class for bi-weekly group chat sessions for kids with special needs (those who found socialising more challenging). Make sure you enrol him in a nursery with a decent SENCO and an empathetic teacher who has experience of working with ASD children.
Keeping a low-stress household and just basically understanding who my little boy is, what he responds well to, what stresses him, and when I can push the boat out further and challenge him more, has all really helped. And having an amazing dog! 
Get fresh air and peace for at least 1/2 an hour to an hour a day.
I did a lot of PECS at home mainly, to teach DS vocabulary, to teach him emotions (faces showing anger, sadness, happiness, etc), and to help him understand first, now, next, last (first have breakfast, next get dressed, then brush our teeth, etc- so that he wouldn't find change stressful- transferring from task to task when he was small was really hard. Now, he's so flexible and easy-going. He just has no issue with change at all).
I found telling him that 'it will be ok', just soothing him when he was stressed was so important, holding his hands and telling him to give mine a squeeze to release the stress really helped (he was very flappy, exceedingly flappy. We used to joke that he'd fly off. At nearly 6 years old, he hardly flaps but he does jump up and down when he's happy). Skechers shoes are your friend, in case your LO has got sensory stuff where he finds shoes and sock uncomfortable. Basic grey school socks and Skechers and soft jogging bottoms are still DS's go-to clothes when at home. But he's done very well at school with the uniform: button-down shirts and ties and awkward Teflon trousers. Never complains.
As your DS gets older and learns to hold a pencil, buy some rubber pencil grips from Amazon. Stabilo easy-grip pencils are great for all kids, but especially helpful if your child is having a tricky time holding a pencil. Stabilo Woody pencil crayons are brilliant for colouring.
You don't have to talk and chat all the time with your ASD child. This can cause stress in some kids, unless you have a really chatty child. My DS didn't speak until he was 3. He is an absolute chatterbox now and speaks really well. Just talk clearly and concisely. Explain how things work. If you do A, then B happens. They like to know the consequences of actions. Sometimes you just need to clearly drive points home, so rather than, "Don't touch the iron" you could say, "If you touch the hot iron, it will burn you and you will have to go to hospital and that will be sad for you."
Lots of hugs and squeezy handholds help relieve stress and release oxytocin, always a good, calming remedy to stress. I am over emphasizing this for a reason. ASD kids tend to be incredibly emotional and love being loved. But sometimes it's that inability to deal with the onslaught of emotions that stresses them. Keep it simple. Keep it lovable and gentle. My ASD son is my most affectionate child. Lots of hugs, as I said. Lots of love.
If eye contact isn't great, use 'eyes to me' or 'face to me'.
ASD kids adapt and learn to cope really well. They tend to do well with rules and lots of structure (try not to be too strict or domineering or this can cause stress).They do learn by example, as do most of us. But what may be intuitive to one child, may be less so to an ASD child, so you show them, guide them, and they learn.
Paediatrics have diagnosed your DS, have they given you a folder telling you what's available or about any local playgroups? We didn't really have anything local to us, but to be honest, a good nursery is key to helping them socialise happily. So choose well.
It's sad and scary, the time leading up to diagnosis, but it really won't always be this way. With early intervention and a lot of love, your little one will really thrive in the light of your love and support. Try not to be afraid. I know that it's daunting. It's as if suddenly your life GPS says 'route not available' and now you're stuck on this road and you don't know where it's leading or what lies ahead. But soon enough, you know this road like the back of your hand. It becomes a well-travelled and well-loved route. And you know which potholes to avoid, having had a few previous run-ins with them.
Sorry to ramble on. I hope I haven't overwhelmed you!