How does my 7 year old son get more Pokemon for this game?

Ok how he can get more pokes for that game is he has to get Pokemon pearl for the ds, Pokemon pearl is a different game then revolution, the ds is a different system then the wii, a ds is $120 and Pokemon pearl is $30, once he has both of thoughs items he can transfer the Pokemon he gets from pearl onto his revolution game, then he can use them to battle people with on the wii, I hope this info helped you.

You need to link with Pokemon Diamond or Pearl and copy the Pokemon, no other way

Buy him a DS and either Pokemon game for the DS (Pokemon Diamond or Pokemon Pearl)
Let him play the game through (help him with some of it, because at seven, he may find it taxing) let him catch a lot of good Pokemon and train them up, and when he's got a fair few good ones let him transfer them to battle revoloution on the wii. There should be instructions on how to do this in the guide for revoloution.

Also, if he would like Pokemon from the other games (sapphire, ruby, emerald, leaf green or fire red) you will need to buy those games, too. They are for the Gameboy advance, but you can play them on the DS as well. Play them all the way through, and in diamond or pearl, you have to have seen all the Pokemon in sinnoh (it's about 150 and it's do-able) then you will be able to transfer six Pokemon a day from any of the five games to diamond or pearl. If you want to transfer Pokemon between any of these games (no to revolution) you will need two DS's for diamond and pearl or two gameboy advances and either a game link cable (will allow you to trade between any of the games) or two wireless adapters (they came free with leaf green and fire red) to trade between LG/FR or emerald) I wont confuse you anymore, suffice to say that to really get full enjoyment out of Pokemon, it helps to cross a few platforms

Or, if you don't like the Pokemon he has, beat the stargazer coleseum and go to the gateway colesseum. You will be able to select trade battle. I don't know about dialga and palkia but I got a lucario from it.

You can copy the Pokemon you catch in Pokemon Diamond, Pearl and Platinum games(DS) to Pokemon Battle Revolution(Wii), but if you can't get those games, you can also play the trade battle(after beating the game) in gateway colosseum, and after every battle, you'll get the chance to trade a Pokemon from your opponent for one of your team, by doing this, you can get your own cuztomized battle pass.
If you play gateway colosseum before beating the game, you can get new battle passes, but you can't trade Pokemon with your opponents. Hope this helps!

you have to get a ds and a pokemon ds game then get alot of pokemon and transfer to the wii game