Well I hate to tell you both this but at these prices the pins you are buying are almost 100% guaranteed to be scrappers...in other words fake pins. The CM's will take them and then someone else will get the scrappers in trade.This is not good advice. However someone else provided the same link I have in my signature and that thread has tons of very very good advice.
That thread has a list of good eBay sellers, people who do not sell scrappers and who do not charge an arm and a leg for their pins.
As for "professional" traders, at times DBF and I would stand at the tables with our book to trade, we did it mostly when the girls were off riding rides or when we just wanted/needed a quick break. All traders that stand at the tables are not out to "shark" you, some of us are genuinely nice people who just enjoy a good trade, so start with CM's but don't be afraid to talk with other traders as well. If someone rubs you wrong or you feel uncomfortable just walk away, but know that there are quite a few of us who do it for fun but still use the tables to display our available pins.
Buy them bulk.
I am currently waiting for a batch of 100 that I got for 64 bucks. Of those, we usually find about 5 to 10 that we like enough to keep.
64 cents a pin puts them right in my price range
The lanyard is just a fun way for him to show them to the world around him. You can keep them anywhere you like. He may find they are a bit awkward for the pocket and want to keep them elsewhere, like in a pocket of a backpack, for example.
My girls sometimes don't want to wear their lanyards so they will carry them in their backpacks, when we put them in our bags to carry for the day (we always have extras on hand in our park bags) we put them in small plastic baggies and then we put those small baggies in bigger baggies, it keeps them from getting scratched up when they rub on each other. 
I tend to pay at last $3 a pin for traders, sometimes more. My best advice is to seriously read the thread in my signature. it's very helpful and there is a list of eBay sellers as well.
Aww, of course he can have them in his pocket.
As for whether or not CMs will trade with you son if you have fakes, the answer is: almost for sure. For a few reasons.
1. They may not know they are fakes unless they are collectors.
2. They would not want to confront your son in case they are wrong.
3. They know that in MANY cases the trader has no clue they have fakes.
For the above reasons, it is often possible to get a fake from a CM lanyard.
Now for the most difficult of your questions: How to tell if they are bad
It's not easy. Here's a few tips:
1. The Mickey pattern on the back. On the newer pins, there is a repeating Mickey pattern on the back. It should run off the edge of the pin, not end nice and neatly at the edges. If it DOES end perfectly as the edges, it's probably a fake. The only problem here is that older pins do not have the "wallpaper Mickey" on the back.
2. Weight. Get used to holding one pin in your hand just loosely. Fakes often feel just too light. Yes, once you have gotten used to holding a single pin, you can tell.
3. This one is tough. If you KNOW the pins you are trading for, one of the key signs of a fake is that the colors are slightly wrong. This is a tough one if you are trading for pins you have never seen before. Examples of this can be found online if you run a search.
The best way to learn about fakes is to do some research. Because of the DISBoard rules against posting links to external sites, I can't put links up, but if you run a Google search for "How to spot a fake Disney pin?" or something similar, you will find a LOT of information.
- Dreams
