I'm going to assume your DS wears adult sizes. Thats where they get you. I would start looking now for bargains on pants and shirts. I don't know if you have a finish line where you live, but every years at back to school time they have adult sized polos 5 for $20 and they are the nicer quality.
My DS is tall and skinny so we have to get most of his clothes at Gap. He wears a 28x34 which are really hard to find. Last year I was able to get several pair on sale at of all places American Eagle.
I know others don't agree with me, but I HATE uniforms. All it has done is empty my wallet and make me do double laundry. The kids never wear their uniforms to after school things. They always take jeans and shirts to change into, so its just double the clothes and the laundry.
Old Navy also sells uniform clothes, but we have found they don't last as long.