clarify which monster hunter you guys playing
i assume u guys are playing monster hunter freedom unite
few possibilities i could think of
1. different channel on wifi, go to settings > network settings > ad hoc mode, you both have to be on the same channel. eg : you and him gotta be on automatic, or channel 1, or any same channel
2. you and him using different version of monster hunter freedom unite. For your info, there's US and EU version, if i'm not mistaken, both version doesn't connect to each other
3. you two are using different monster hunter. eg: 1 using freedom unite another using portable 2g(which i doubt this could happen

)
well we checked and the MHU we play definitely same version coz he took the game from me. Exactly the same one.