CLASS - A UNIFORM
Shirt:	Official Dark Blue Shirt (short or long sleeves) w/ Patches Properly Affixed (Some Webelos dens wear the tan Boy Scout shirt.)
Pants/Shorts:	Official Dark Blue Pants or Shorts
Belt:	Official Belt and Buckle
Neckerchief:	Official Neckerchief (yellow for those working towards Wolf, blue for those working towards Bear, and the plaid for those in Webelos) Cubs may wear a specially designed neckerchief for all local activities and ceremonies. Cubs should wear the "Official" neckerchief when participating outside of the local den or pack.
Socks:	Official dark blue socks w/ gold trim. Any of the various styles (tube, crew, or ankle) may be worn. For local and pack events, we do not require the official socks.
Hat/Cap:	Official baseball style had with the yellow front for Cubs working towards Wolf, or the blue front for those working towards Bear, and the plaid front for those in Webelos. Wear of locally designed hats/caps is authorized for local den and pack activities and ceremonies. When participating in activities or ceremonies outside of the local pack or den, the official hat/cap shall be worn. While indoors the hat/cap shall not be worn, unless the wearer is posting the colors, providing ushering service, or involved in other ceremonies/activities that require the wearing of the hat/cap.

How to properly wear the Class - A Uniform.

CLASS - B UNIFORM
Shirt:	Official Dark Blue Shirt (short or long sleeves), Pack T-shirt, Den T-shirt, other official BSA activity shirts, or T-shirts from camps attended by the wearer.
Pants/Shorts:	Any pants or shorts neat in appearance and appropriate for wear for the scheduled activity. This includes blue jeans.
Belt:	Official Belt and Buckle is optional.
Neckerchief:	Official Neckerchief if wearing the official uniform shirt.
Socks:	Optional wear of the official socks but not required.
Hat/Cap:	Official baseball style had with the yellow front for Cubs working towards Wolf, or the blue front for those working towards Bear, and the plaid front for those in Webelos. Wear of locally designed hats/caps is authorized for local den and pack activities and ceremonies. When participating in activities or ceremonies outside of the local pack or den, the official hat/cap shall be worn.

CLASS - C UNIFORM
Shirt:	Pack T-shirt, Den T-shirt, other officially BSA activity shirts, T-shirts from camps attended by the wearer, or other attire prescribed by the Den Leader.
Pants/Shorts:	Any pants or shorts neat in appearance and appropriate for wear for the scheduled activity.
Belt:	Official Belt and Buckle is optional, but should only be worn if a Scout type shirt or T-shirt is worn.
Neckerchief:	None
Socks:	Not required.
Hat/Cap:	Official baseball style had with the yellow front for Cubs working towards Wolf, or the blue front for those working towards Bear, and the plaid front for those in Webelos. Wear of locally designed hats/caps is authorized for local den and pack activities and ceremonies. When participating in activities or ceremonies outside of the local pack or den, the official hat/cap shall be worn.

The proper Scout salute shall be rendered at appropriate times when wearing any of the above uniforms that include either the Official Dark Blue shirt or a Scout T-shirt. If the Cub is wearing casual clothing, then he should place his right hand over his heart for patriotic ceremonies.
I have found many uniform items for sale on eBay for less than one-third of the cost of new uniform items. For instance, I purchased both of my brand new official shirts for less than $18 each including the shipping costs. I picked up two pairs of official uniform shorts, each at less than $15 including the shipping. A new pair of uniform pants for me cost about $40, but I picked up a brand new pair on eBay for under $25 including shipping. If you do not use eBay and would like for me to keep an eye out on good deals for you son, let me know.
Another good source for uniforms is a thrift store. I have seen shirts and pants that appear new for just a few dollars.