Then, look for options that will "break" his strenghtstrength, without hurting the others too much. There are a few options for that, based on the Knight class:
Spells that grant negative conditions, such as shaken, nauseated, entangled, exhausted, or sickened, are all good options, as some of those will lower his strenghtsstrengths and allow enemies to have a better chance against him.
If you really want to see him rage from a failed save, make an enemy cast Create Pit on him. And watch him strip after a bunch of failed attempts at climbing up (rememberremind him that he has a -8 penalty from armor).
Knowing this, you know should know that touch attacks will be checks against the character unarmored AC (10+ dex bonus + dodge bonus + deflection modifiers usually), which is much lower than his armored AC.
If he built a character around defense and melee attacks, a flying enemy will be way out of his reach for most of the time, unless someone on the party casts a Fly spell on him. But even with the spell, since he wears heavy armor, his 40 feet flying speed should be lower than most creature'screatures.
As iI mentioned, he wears heavy armor, which means that unless he takes a lot of levels on the Fighter class, his speed should be lower than average (20 feet or so). Difficult terrain should be a nightmare for his character, as he will move only 10 feet per move action.
While grapple will force him to not move towards other enemies (like their boss which happens to be a spellcaster or ranged attacker), it will also reduce his AC (-4). If he is tripped and then grappled, his AC should be much lower now. Just keep in mind that, unless the attacker has reach or the proper feat, they will take attacks of opportunity aswellas well. But since his damage is low, they should probably be able to take a hit or two and still trip/grapple him.
This can be used on attack rolls, raise the AC of their boss, help tripping or grappling an enemy, and whatever else you can think of. The cultists you linked have sneak attack and the command spell. I know that their sheet says that they will only use command on emergencies, but a towering enemy wearing a full plate armor should probably be a higher emergency than saving it to escape. Commanding him to drop prone will help them to grapple and probably pin him in maybe two rounds. You will have to consider their intelligence (10), or if their boss could come up with that idea on his own (14 int, probably yes) and tell his henchmenthenchmen to do it.
For their boss, his AC is actually on par with your knight. His suggestion should be used as soon as possible on the biggest and dumbest enemy he can see charging at his beloved cult, while his invisibility is a terrible idea to use at the start of combat (despite what the developer said), as most of his other options are offensive and will be wasted casting early on. Instead, he should use his copycat as often as possible (he has 5 uses and they last 6 rounds), to prevent attacks from hitting him. On top of that, his Reaper's Mask allowallows him to cast the Confusion spell twice per day, with a DC of 16 (minimum to cast the spell).
Again, all thisof these are simply hints on how to make combats difficult for him. Remember that the objective of the game is to have fun, so you create challenges that they must overcome. If you, as the Game Master start to play against your players, they will get annoyed or worse, and might not come back next session.
So don't abuse on these extra challenges, make his character shine in combat once in a while and he will have fun with the character he built. Throw a Stone Giant against him so he can brag about it ("I survived the blows of a giant!").