teaching SJ to not be a poor sport

Last night, SJ, Bob and a neighbor boy were having races in our backyard.  Most of the time, Bob won.  He is older than both SJ and the neighbor boy.  Occasionally, SJ won.  SJ was coping well most of the times that he lost.  After about 20 minutes of losing, he was getting frustrated.  When SJ gets frustrated at losing, we see several behaviors.  He starts to shout at Bob.  He may say that it is not fair or it was all Bob's fault.  He may even declare that he has won when clearly he did not.  He may also pick up the toy and throw it.  We have been trying to work on stopping this behavior.  We tell him that he is being a poor sport.  We tell him that he can't yell at other kids (even if it is his brother) and he definitely can't throw things.  We tell him that kids won't want to play with him if he acts like this.  This is why he is currently not playing any team sports, like baseball or soccer.  My husband and I have been trying to recognize these behaviors and stop them before he gets too mad.  We pull him aside and take him inside to stand in the corner.  He hates this punishment, but it does seem to be effective.  Sometimes, the simple threat of standing in the corner is enough to stop his yelling.  The problem is that after his punishment is over he will choose to go upstairs to be by himself.  That is defeating the purpose of him playing with Bob and other kids.  I wish there was an easier way to get him to remain calm when he loses.  I guess we will just have to keep at it.  Practice makes tolerance!