I'm pregnant

Hi I've just found out that I'm pregnant. It's a big surprise to say the least! My hubby and I have 2 kids aged 10 and 7 and were done having anymore! My hubby is especially against having anymore children as I went through a time there 2 years ago of wanting another baby and he said definitely not never again. I accepted that and realised 2 was definitely enough. We are always careful and use condoms as bc. He never comes near me without one! Yet here I am 4 1/2 weeks pregnant. HOW CUD THIS HAPPEN?? I'm so scared to tell him coz i no he will be angry please help. How do I tell him im pregnant so he doesn't freek out???