at that age its up to you. if you dont want them to do it, you dont even have to mention it. at that age they dont notice what others are doing. but if you want him to get into it you can certainly convince him its fun and coax him to do it.
we just got back with our children 8,4 and 3. the 8 yo has been pin trading for years. before we went my 4yo said he wanted to trade this year and i was all ready to buy him a starter set, but when it came right down to it, he just didnt care all that much. he would rather i spent that money on legos and such. i could have pushed him, but i didnt see any reason to. my 3yo, he didnt even know it was going on around him. even watching his sister, he didnt care.