Temper Tantrums

I have a 3 1/2 year old and a 5 week, and as expected, our little boy has become really naughty. He has to have his own way all the time, has to be first at everything and will not do as he is asked. If we are out shopping and someone goes through the automatic doors before him, he will shout and blow rasberries at them. The last couple of days however, have got worse, and he has developed a really bad temper. He hits out at me, kicks me, scratches me and is really cross all the time. I know it is a reaction to his baby sister and it will pass, but I wondered if anyone had any tips on how to get through it. Thanks