Maybe help him understand a few things a little deeper, such as: Explain that sports are a physical activity that often have physical contact. But because his teammates and him are still learning much about the sport, accidents happen more often. The other kids aren't TRYING to hurt him, they are just trying to play, learn and practice all at once. And all his teammates are so eager to get better, that they all want to have the ball so they can improve that much faster. Or also try having him watch a proffesional game on tv or even go the the local high school and watch a game and point out how often the players get bumped into and pushed. Then ask him to imagine those big tough guys getting upset everytime it happend...the game would never end ;)
Maybe have him really pay attention to other players while he is not on the court. Tell him to watch the other kids who don't have the ball and maybe it won't seem like he's the only one who doesn't get the ball as much as they'd like.
You could also talk to him about how much wiser he is since he knows when something is not fair and if he chooses to do the right thing back (instead of reciprecating the bad behavior) he has already learned more than that other child and he is now setting a good example for the other children to learn from.
If the coach is letting certain members of the team "hog" the ball, then I would express my concern to the coach that teamwork is a crucial lesson at this age. As long as you are doing your part to help your child, coach should be doing his/her part too. My son's basketball team had a "star player" and the other kids on the team liked to pass him the ball so he could score (and they did that a lot! pass it, not score) but coach always reminded them to give equal time with the ball and include all teammates. They may not have fully learned the lesson, yet, but at least coach was being consistant with teaching it.
This might be weird, but maybe tell him that everytime he is open and doesn't get passed the ball to count that as 1 private point for him. He can kind of keep tract of how many points he could of made for the team, but since they didn't pass the ball he got the point. You and him could secretly "keep score" and give him his own little celebrations on how well he "did" each game. If he pushes himself to have more private points, where he really was open, then maybe the other kids will start noticing it and start passing the ball. And maybe give him some strategies for dealing with his emotions on the court. Like, if he wants to cry, bend down and pretend to be fixing his shoelaces while he takes some deep breaths and doesn't let anyone see his facial expressions while he tries to quickly process those emotions. Or bend over and rest his hands on his knees to look like he's catching his breath, but instead is taking deep breaths to calm down. I hope a little of this info is helpful.
I really feel for you, I,too, have a sensitive boy, who struggeld (for him it was with poor sportsmanship) for years. He still has his momments (at age 9), but has really changed a lot with age and experience.
ADDED: I think punishing him for his reaction is wrong. I would try and give as little attention to it as possible (like ignoring a tantrum). I think he would then notice his teammates and other peoples' reactions to him more. I don't think he'd like the reactions and could choose to react differently. You could give a quick, direct statement about how he could respond next time (like "Try giving Mom a good, solid High-five next time you get upset like that...let some of the frustration come out of your hand. Besides, you'd look pretty cool doing it"). And as far as rewards...heck ya! These are the learning years and children need tools to learn....positive reinforcment is a great method. But, I do have some advice here-instead of you doing all the praising, work on him validating the praise. Such as "Aren't you proud of yourself?" or "Wow, what part of the game do you think you did your best?" I don't think parents pay enough attention to their children's self-esteem. One day I asked my son "Do you love yourself?" and he said "no." I wanted to burst into tears. At the preciouse age of 6 he had already been beaten down by life enough to have self-doubt and conditional love. I have worked very hard since that day to make sure he knows that he is exactly as God intended him to be and he intended him to be perfect! He is 100% unconditionally lovable and acceptable no matter what he does. Actions and behaviors can be bad or wronge, but children themeselves cannot be bad or wronge. All God's children are created equally! It's our free will and choice that can be corrected, but our soul's need no fixing, they're perfect and beautiful. Life can be answered by 2 questions: Are you enjoying it? If not, are you learning from it?