What is wrong with my cat? He's acting weird?

I've had my cat Socks for 6 yrs. For the last year he has been an only cat because his brother died. About a month ago I decided to get him a playmate because I know how much he loved his brother. It took 3 weeks for them to get along. He was very stubborn. So, for the last 2 weeks Socks and the new kitten, Boots have been great buddies. But today something happened. For some reason that I have no clue about, he starting growling and hissing at the new kitten! They stay indoors and Socks is neutered, Boots isn't old enough yet. I got home about 5:00, my kids wanted to play outside so I left the door open and the 2 cats were just sitting at the door looking out. About 30 min or so later, Socks ran under the table and starting getting very aggressive towards Boots. Socks always gets upset when he sees a strange cat outside, you think he saw a cat outside and triggered this? I don't know what to do. Its really bad. He is hissing and growling as I type. Boots, the kitten is staying away from him because he knows something is wrong with Socks. Any advice?