good luck in YOUR trying to understand it, however your son will likely develop an encyclopedic knowledge of it in a relativly short time.
my ds has been ADICTED to pokemon and yugioh for several years (he's 10) and as best as i've been able to understand-pokemon means 'pocket monsters' and they can be used for good or evil. different pokemon have different 'skills' and attributes, and in playing (with the cards) the player calls upon these to battle against their opponants. yugioh card dueling is much the same in that different cards can and can not be used in battling your oponants.
as far as the books go-there are allot of books for his age range that are short early reader books that are basicly like the cartoon shows, they tell a story of a particular event in one of the character's lives. there are other books-like guides that are references for what can do what to whom and offer strategies. then there are strategic books about whatever the current popular video game is-because there will be the game, then the associated release of the cards associated with that game.
i tried watching a couple of ds's videos-but they were too confusing for me, dd (13) was able to develop a basic understanding-but only dear old dad could realy understand it all in our house, so ds and he end up 'doing battle' on my dining room table (as i speak it is covered with cards and a dueling mat
).
a couple of suggestions-for the movies, keep an eye open at garage sales. i've found that while kids and teens hold on to thier cards forever they tend to let moms sell off the videos and dvds. i've also found allot of the books at goodwill and garage sales (at least the story type ones).
unless you locate a source for bedding or clothing or posters don't indicate to your son that you'll get some, they can be very hard to come by (the cards are still very popular, but associated items are not always easy to come by). my ds wanted a yugioh themed room-i went to walmart, target...no luck. ended up on the internet with some bedding company who sold me their last comforter/sheet set in stock (and they said it was no longer available). posters often come in the crazy overpriced magazines. tshirts and such-since for some reason despite the continued craze with young boys the stores are'nt carrying them, i've found some used ones in good condition at goodwill.
as for the asd-interesting you mention this. at the last neurologists appt. ds went to, a boy noticed ds was playing with his gameboy advanced which was a special release at toys r us and looks like pikachu (one of the pokemon characters) and began chatting away with him. the mom and i chatted and she mentioned that her son was autistic-and that she was so happy when he got into pokemon, that it seemed to provide a universal basis of conversation for him to share with other kids-and that she had seen his social interaction soar based on him interacting more, and interacting successfully.