Slayers don't have a whole lot of options that are explicitly anti-magic, especially on humans (dwarfs get a handful of related feats). That and PF in general doesn't play well with mundane anti-mage tactics in general.
There isn't going to be a silver bullet in terms of options but I'll give it a shot. Feat wise, investing in step up/step up+strike is a moderately worthwhile investment since that at least will force casters to work defensively when engaged rather than just shimmying away. Grappling is another option that's generally able to easily shut down non-monster casters but the prerequisites are abominable on a slayer, especially now that he's L2. Improved Init is always welcome and going first is a great boon vs casters since it allows you to get up in their grill before they can start playing havoc on the battlefield. Other than that, just get anything to improve damage output since the best type of anti-magic is corpsing the wizard ultimately.
Gear wise, he'll want options to bypass the usual mitigation tricks (darkness, flight, blur, mirror image, etc) although these are not something to worry about for now since the costs for those tend to be high to say the least.
