Soccer is a game that teaches life skills. It is a place for children to learn to be better people. Do encourage your child to play fairly. Things happen on the field that can be construed as unfair. The ref may miss a call, there may be a careless tackle. Whatever the situation, you child and you should not retaliate.
Do encourage your child to give it everything he can while on the field and in practice. Make sure your child is ready to play when he arrives at practice and at a game. That means you need to make sure he is fed and hydrated. It means they are on time for every practice and game.
Do encourage your child to be a good sport. Not everything goes your way. Sometimes it seems that all the calls go against you. It's hard, but the reality of the situation is there will be games where the call go against you and games where all the calls go your way. If you complain when they go against you, you should also complain when they are all in your favor. In either case, it's not fair to someone.
Do encourage your child to express gratitude to coaches and referees, regardless of how the game turns out. Your child should be grateful he is playing a game he loves. It isn't about the "W" it's about the process of becoming a better player and a better person.
Do encourage your child to be kind to everyone. Regardless of the situation, he should be kind to everyone around him. That means the teammate who missed the shot or the GK who had a bad day. At some point every player on the team will have a tough day and your child's kindness can make a team better.
Do encourage your child to be the type of person you hope they will be as an adult. Soccer is a microcosm of life. How he behaves on the field will impact how he behaves in life. Also please note that your child is looking to you as his example. Act like your child wants you to behave.
It all starts right now.