I know a very good counter: Bisharp!
Bisharp @ Life Orb
Ability: Defiant
Nature: Adamant
EVs 252 Speed / 252 Attack / 4 HP
-Swords Dance
-Sucker Punch
-Iron Head
-Knock Off
So, your bro send out his Mega-Alakazam, and you send out your Bisharp. As a whole, Bisharp resists probably every move that you would expect an Alakazam to have, as Bisharp resists moves like Psychic, Energy ball, and Shadow ball, and Dazzling Gleam deals neutral damage. This Mega-Alakazam out speeds everything, but since you know it doesn't have a focus sash (since it is a mega) and because you have Sucker Punch, and because Mega -Alakazams tend to always have attacking moves, you could just OHKO his Mega-Alakazam with a simple Sucker Punch or you could Swords Dance in case his switches out.