Are you coaching the soccer? Unless you are coaching, you need to stay out of it. This is between your son and his coach.
From what you have described, I can tell you this right now: Your son is NEVER going to be a star soccer player. Are you okay with that? You can tell which kids are going to be superstars on the playing field by 6 years old. They have the competitive drive, they love to play and practice, they don't give up, whether they win or lose, and they have natural ability.
And you know what? It doesn't matter if your son isn't a star player. He's doing something, he's getting exercise, he's learning something.
If he cries and complains to you about losing, give him a short, pithy statement about winning and losing, and about losing gracefully, etc., and then don't listen to him complain any more. Losing (failure) is a necessary part of life, and he needs to learn to accept it.
If you are the coach, then I am not going to advise you, because I have never coached soccer (although I play). I have heard, over the years, that coaches kids are usually PITA's to their parent coaches. It's always better not to have to coach your own kids, unless they are natural superstars.
Don't over-analyze this. You can't "make" him feel defeated, and he needs to learn to accept loss and failure, because those things are normal.
And as the others suggested - try him in other sports. All good advice below.