What else other than cash could a 14 year old use?
But yeah, if he's got the eye for being a bit unconventional, it shouldn't be too hard. Between dragging clothes out of the back of the closet, the craft store, thrift shops, garage sales, and the numerous DIY sites on the internet, along with some dedication to the project, it isn't too hard to improvise while making clothing. I've done that part enough times on my own to know that it's true, even if my skills as a seamstress are horrible.
On hiding while making itů well, the night and a flashlight are your friends as long as you have a room to yourself without the risk of someone looking in. If that doesn't work, its fairly easy to shove the costume in a trash bag in the closet, put it in the bottom of a backpack before school, and find a secluded spot ether at school or afterward, like in a one-person bathroom or something.
Making sure it won't get back to him ever? Hmm. Well, I assume that people won't bother to notice a kid going and buying clothing, as long as it looks like something that will fit him. You can cut that stuff up for material fairly easily, and as long as it doesn't look
like the original article of clothing, it should be fine. If his mom sews, she probably has a sewing kit, and it should be pretty easy to nick a needle and some thread. There's also things like dying clothes darker, which I think you can actually do with some coffee, hot water, and a little time, but then there's things like koolaid, bleach pens, and actual fabric dyes too. Hell, just look into how people alter their clothes on their own now, and you should get plenty of inspiration on how to obfuscate the materials he's getting.