OP 
Who are you on the waiting list with? SN support and provision is such a postcode lottery and in many areas of the country you need to be a pest to get any. There's a saying that the squeaky wheel gets the grease, start being a squeaky wheel. On Monday phone whoever it is and chase up the support referrals. Also give your Health Visitor a call and ask her what support she can offer in the short term, ask her to chase up the referrals on your behalf too.
When he's playing are you down on the floor with him? I know it's not easy especially with a newborn and other children (I have four myself) but when DS was younger I found that being on the floor with him, with him sitting between my knees if need be, lessened the behaviour a little bit as he liked having me close by so was calmer. I was able to sometimes stop him from hitting before he hit but the other times, when I couldn't stop him in time, I could stop him from hitting more than once and could immediately move him away.
Longer term, something that helped was to keep a sort of log of incidents. As soon as possible afterwards I would note down what happened, time of day, place, what he was doing before it happened, what happened afterwards, how long it took him to calm down, what helped, what didn't help, etc. It helped me look for patterns to his behaviour and to spot potential trigger points so I could avoid them in future. It wasn't perfect, DS will still sometimes act in ways I can't predict (he likes to keep me on my toes), but it did help me spot some patterns that I couldn't quite see while I was in the thick of it.