How do we get him to stop crying?

Our 5 Yr old crys about everything!!!!! If I ask him to come to dinner he crys cause he wants to play with something. He crys because I did not use the bread he has in his hand after I've already made his sandwich. He crys when I ask him to share. He crys when we ask him not to cry! We don't know what to do! He just gets sad alot!!!! All the time! He is only really happy when he is playing on the computer, on his ds or outside playing. And we have 3 kids so I can't take away the computer and ds cause our other two wil play and he will just cry. It's like he does not understand why... Like you actually feel bad for him because it's not like a temper tantrum; he just gets really sad, confused and crys! Anyway I want to stop this behavior. Right now we just put him in timeout wich makes him stop crying almost immediately but he then has to sit it out (5min/ age) he does Not like time out so everytime he is about to cry we say you will go to timeout and usually he will stop, but I don't want him to even start to cry!! Any advice??