DS is 4 1/2 and up until the last couple of months I've never had any real difficulties with him. He's always been a lovely little boy - intense and very demanding of my attention at times, but also sweet and funny and affectionate.
Over the last few weeks his behaviour and mood seem to have changed quite dramatically. He's been sulky and rude, and at times quite defiant - really testing the boundaries. He seems angry, with me in particular, but also very needy, as though he wants me to focus on him all the time. Sometimes he gets into a sort of manic state where he's hyper and over-excited, and there's a lot of charging round and bashing and shouting and silly voices.
In the last fortnight or so he's started to have proper melt downs where he just can't seem to control his emotions. This is normally triggered by me saying no to something, or giving him a gentle telling off for some minor misdemeanour. He gets incredibly angry and quite aggressive, trying to hit or kick me and throwing things around the room. On a couple of occasions he's said that I don't love him, that everyone hates him and even that he hates himself 
DD is 7 months old and I'm wondering whether part of this is a delayed reaction to having a new sibling? For the first few months I thought he was coping brilliantly with the change, and has been very sweet with DD - but recently he's made comments along the lines of how he'd wish she would go away because he wants to play with me.
I have no idea whether I'm handling all this in the right way - this kind of behaviour is new to me and I feel out of my depth. When he gets angry I try to stay calm, reassure him and encourage him to talk about feeling angry rather than lashing out - but I also want him to know that it's not OK to hit me, and I can't have him wandering round the house destroying things! I'm worried that if he gets really out of control he's going to hurt himself or DD.
Any advice? I can't bear for him to feel this sad, and I feel like I don't know him at the moment - I just want my sweet little boy back