The fear of losing will fade over time if you explain that everyone loves to win sometimes and the fun of playing the game and being happy for other people is part of what makes playing important..no one likes a bad sport...everyone likes him if he wins or loses..blah blah etc. Unfortunately, you can not speed up that process or make it stick immediately.
When I was little, I HATED all competition and games. I hated losing, I hated winning because other people's feelings were hurt, I hated the whole friggin thing. I liked playing piano-but hated recitals-loved gymnastics-but hated meets-liked soccer-but hated games...to this day I'm not competitive. But I was always polite and nice, and luckily wasn't always forced to participate.
What you CAN address immediately are the tantrums he throws. This should not be allowed whatsoever in an 8 year old child. He should have very clear and firm consequences for beginning a tantrum no matter what the reason. Once he learns to stop this, you can address other things. For example when my 4 year old first becomes upset about something, and tries to tell me in a frantic way, the FIRST step is reminding her to settle down, which is easy, because tantrums have never allowed. From there on, we can address the issue.
You need to warn him in advance exactly what his consequence will be if he throws a fit, and follow through ALWAYS at home, and even if you have to leave an event to do it. NO matter what a fit is for, he needs to know it is never allowed, so he doesn't suddenly do it in these situations. Until he has this mastered, I wouldn't even allow him to play in public situations. He can earn the privilege to participate when he assures you he will be nice. Firm discipline is key.
Teaching him firm rules is not more mean that depriving him of experiences by leaving him home. Teach him how he needs to behave, and address the "love of competition" later. I would also get a hold of some books and movies for kids with good sportsmanship in them so he hears positive examples of how to handle things. Good luck!