Ouch! I feel your pain. I recently attended a college softball game primarily to watch a friend of the family who is a freshman outfielder. She was a hotshot player in high school, and I believe she assumed that the same would be true in college. But she was not in the starting lineup on the day of the game. Luckily for us, her mother had warned us that she hadn't been starting much, so it was not a complete surprise. We spoke with her before and after the game, and we didn't mention not starting (she did pinch hit unsuccessfully). Her team lost the game, too, so we couldn't concentrate on the most important part of team sports--the team's success. If your boyfriend's team won their game, you can at least use that as a starting point in your conversation. I know it's tough for young people to get past the importance of not starting--sitting on the bench--but hopefully your boyfriend recognizes that he probably isn't as good as the rest of the starters and has a good attitude about it. I coached baseball for a while, and I had several players quit because they couldn't stand not starting. Sadly, none of them put the team first; instead, it was all about themselves. Tell your boyfriend to keep his spirits up and that you'll be there for all of the team's games, and stress the part about "team" sportsmanship. He'll probably get his chance to play, and you'll want to be there to see it.
Eventhough your boyfriend did not play, I am sure he was happy that you came. When you come and support the team, you are also supporting him.
What is important is that he asked you to come and you did. Be positive and tell him that you enjoyed the game and you are looking forward to seeing the rest of the games. He may not always get to play and even if he does he may not always win. The idea that you are supporting the team will be most remembered and he is a part of it.
I think when a player of any age knows that there is someone out there watching they feel good overall. You can always say "When is the next game, or that you will also bring someone else with you to the next game.