Mom worried about crying children

I have a 4 1/2 yr old son and an 8 yr old daughter. Both are great kids except they cry at the drop of a hat and my husband & I are at the end of our rope! My daughter is getting better as she gets older but my son cries because his pants are too short or because he's not as good at Super Mario Wii as his friend. I try to tell him that it doesn't matter and he will get better with practice but he doesn't believe me and says he'll never be good. If he gets into a fight with his sister and we have to step in they both start to cry. When they get angry they end up crying. When they are frustrated they start to cry.The other day I was going out to the store with my daughter and my son was going to stay home with my husband. When I told my daughter to get ready I looked over and my son was sobbing. I asked him why he was crying and he said "because I'll miss you" and he said he wanted to go with us. I said no, he had to stay home with his dad and they would probably watch TV. He's trying to stop sobbing and wiping away his tears as he says, in this sad little voice, "but what if I start to cry?".I mean he's already thinking that he's going to cry and nothing's happened!I know we shouldn't get mad, but we've just had enough! Any comments or suggestions would be greatly appreciated.Regards,Mom who's really starting to worry