Wi Fi for Laptop

I have a laptop that I can plug in anywhere in the house and connect to WiFi through Airport Extreme, however, when my grandson comes with his laptop, although it shows my Wi Fi title as his wireless connection and starts to connect, it won't complete and he can't get access. He has been able to in the past, but since installing AE no longer possible. Any suggestions on what might be wrong here and how I could fix this, always remembering that I am a computer dunce, He is 10 and plays a lot of games, and while I don't mind him using my IMac for some things, online games are out. Thanks for assistance. GaelK:confused:

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