He not only played peewee soccer. He now is the captain of his third grade soccer team. You should have suspected he would ask for a soccer birthday party at some point in his life. Better now, than when he gets older and becomes particular in his soccer party supplies.
One option is to find a party supplies store. Since many kids are really in to sports, they tend to carry a wide variety of soccer party supplies. This will make a perfect place to start in searching for favors for the soccer birthday party. Hopefully the kid made this decision early enough for the store to order the soccer party supplies, for his soccer birthday party, if necessary.
Of course, the life of the soccer party is the birthday boy. You could ask that those invited to the soccer party come dressed as soccer players. Perhaps, the birthday boy could get an early present of a new soccer outfit.
For soccer party cake decorations, you could use golf balls and paint black spots on them to resemble soccer balls. If you have an early, enough request for a soccer party, most bakeries do really nice jobs. Black and white are pretty basic colors, so creating a soccer party cake would be fairly simple. You would need basic knowledge of how to use the frosting to it fullest to meet your ideas.
Black and white crepe streamers are also common colors and would accent the soccer party quite well. You also have the option of approaching the artist in the family and asking them to make soccer balls to hang. Supplies would be simply white poster board and a black marker. Even if they are not the most talented at drawing circles, it is the thought that counts. I certainly could keep the siblings occupied allowing you to focus on the birthday recipient.
Another suggestion would be to have the soccer party outdoors, if possible. It would depend on the weather in your area for that day. Assuming it works out, perhaps a game of soccer would make the birthday boys day. If not, it would be an excuse of kicking the shins of the bearers of gifts he did not care for. Scratch that, you forgot to purchase Band-Aids. That probably should be included in the list of things to buy. You may even find that they have Band-Aids with soccer balls on them. I can just imagine all the cuts that are invisible to the eye that will need to be bandaged.
There are many things to take into consideration when planning the soccer party. If you need a laugh, imagine granny in a soccer outfit. We must not forget Spot either. I am sure he would be the life of the party sporting around his soccer referee vest. You almost forgot, what are you going to wear? I guess you will have to start digging in the closets to find something, because your budget does not allow for any purchases of this sort. You spent your entire allowed budget and then some preparing to give your kid what he requested. Although you can just make the choice and tell all the guests to come in their favorite soccer teams outfit, and then have a football match in a field somewhere. That is a true soccer birthday party!