Alright, since I don't know if he has Wi-Fi or not, I'm just going to say this. >_>
First... do these steps...
Boot up your DSi, and then check the System Settings, once your in there, go to Page 3 and tap Internet, then tap Connection Settings.
From there, select Connection 1 (Or whatever Connection # you want.), then select Search for a Access Point. Your DSi will then look for any Wi-Fi Points, if any are found, you should see these Locks.
Yellow Unlocked -> Normal, allows both DS Games/DSi Stuff to go online.
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Yellow Locked -> Password needed, mostly a WEP, once you figure out the password, allows both DS Games/DSi Stuff to go online.
If any Gray Unlocked or Gray Locked icons show up for the connection, then you need to go to Advanced Settings and setup a connection with it.
(CAUTION: Advance Settings only works with DSi stuff, such stuff is the DSi Shop, the Browser, etc. DS games will fail to go online with a Advanced Connection. Gray locks need a WPA Key to allow access.)
If by any chance nothing shows up, or you just can't connect, then... well, I don't know what to tell you.
As for my Wi-Fi, I have a Wireless Router, and I had to config it to allow WEP keys so I can connect with my DS/DSi stuff., I used to use a Nintendo Wi-Fi USB Connector so I can connect, but since I have a Wireless Router... yea. (It was my sister's Wireless Router, but I had to hack into it to change the keys from WPA to a WEP, she found out about it during a later time, but then asked me to just give her the current WEP key so she can connect with her laptop.)