When your daughter was born, he was too young to really imagine how his life would change once another baby arrived. Now he's older, has more experience and a bigger imagination, and it probably feels like the love of his life is abandoning him. It's about the same way most people would feel if their spouse or girl/boyfriend said, "I love you deeply, with all my heart, but I also want to be married to/date somebody else." He can't really imagine how your love will stretch to include everyone.
I'm the baby in my family, so I am still surprized by how strongly (and how often) kids can resent their younger siblings, even if they adore them. My older two are grown and on their own, and still sometimes get resentful when they hear about a trip or party we've had with either younger child.
This will eventually pass, but it'll need your help to pass more quickly and easily. Make sure each child gets 1 on 1 time with you - and their dad. Talk about all the great things about being a big brother. Tell him how much you will, and already do, rely on him. Try not to show him how exaspertated you are, because that will fuel his fear that you'll abandon him. Offer a reward if he can separate without the tears for a certain period of time (start small: say, 10 or 15 minutes.)
Also, make sure nothing else has sparked this fear that seemed to come out of nowhere. Did someone he loves move away or die? Did someone he knows just divorce? Does he go to preschool or a regular babysitter, and has something happened there?
Good luck, Mom! You'll all be fine!