A.V.
Many sites have polos in uniform colors. Generally the colors allowed are white, blue, black and navy. You should confirm this. Google for "uniform shirt". Even Children's Place has gotten in on the uniform bandwagon. It may be harder to find neon colors in a polo that fit the criteria. I would steer him toward other colors if you are unable to find neon.
Old Navy has a lot of colors if he wants and can have more than blue. http://oldnavy.gap.com/browse/category.do?cid=1015532