Just some ideas,
phil ...
1.) Battlefield control: limit his chances for cavalry charges, or provide environments where he can't get on wolfback. Caltrops, marbles, kobolds on top of walls with spears, tanglefoot bags... even something as simple as having to climb up a ladder.
2.) Magic: have the party encounter spellcasters who don't try to whittle down their hp but go after their minds with illusions, mind control spells or simply thins like
cause fear. If he's a cavalier, his Will saving throws probably aren't very good, unless he pumped up Wisdom during character creation. Also remember that a lot of direct-damage spells resolve their attacks against touch AC, meaning that heavy armor won't save his bacon all the time.
3.) Ability score damage: introduce enemies that cause ability score damage. Something as simple as a pack of giant spiders whose venom causes Str damage can be a nice way to make him think twice about charging right into the fray. Web him with multiple spiders, then have them chomp down for multiple doses of spider venom. He apparently has a decent Fortitude save, but all it takes is a single bad roll...
4.) Reach weapons and ranged weapons: take a lesson from history, and use the weapons that were the bane of armored cavalry. Or if he himself favors reach weapons, throw in a lot of combatants that get on his skin, thus denying him the use of reach weapons.
5.) Firearms: campaign-dependant, of course, but even rudimentary firearms in Pathfinder resolve attack rolls against touch AC, effectively making a heavily-armored cavalier very, very squishy indeed.