So many of us here know exactly how you feel. Well done for getting through today - and it's good you've finally seen someone. I know it is really frustrating having to wait even longer now till your next appt, but there is lots you can do to help your ds while you wait.
Know exactly what you mean about doing something (that's why we started ABA even before ds had a dx). The Hanen books will definitely help.
Some other thigns you can do to help make daily life a little bit less stressful, and to help ds with communication:
Use visual aids as suggested before, and use them as well as the word when you say something (eg say 'drink?' while you hold up his cup)
Keep language very simple - no unnecessary words in sentances, don't repeatedly use his name in a sentance. To get his attention when you're talking to him, go up to him and in his vision, and down on his level, when you speak to him.
If groups etc are stressful for both of you, don't go! Socialising is important to children but he might not benefit from that aspect that much anyway as the groups are ime so noisy and chaotic so may just overload him (hence the tantrums). If you can, try and organise smaller playdates with your friends with similar age children. Outdoors especially, so there's no 'pressure' on him or you to keep things calm!
Whenever he does any kind of communication or interaction which is what you want (even small things like coming when you ask, looking at you when you talk to him, giving you something, playing with you, engaging with you or others, etc) give him massive rewards for it - whatever he likes best (food, drink, tickles, throwing in the air, whatever). It's the basic Supernanny principle - ignore the 'bad' (I don't mean naughty, I mean inappropriate), and praise the 'good'. The message is that he gets more of your attention/fun/rewards from engaging with you than he does from not doing so.
I really would recommend the ABA books mentioned earlier, too.
But for now, give yourself a break - I hope it turns out not to be the case, but your world might be turned upside down from how you imagined things to be, and esp with a young baby to look after too, you need some looking after too. Take as long as you need to cry, have a big glass of wine (or whatever) tonight. As someone said earlier, imagine a group of internet strangers giving you a big hug - we have all been there.