How to make the paladin fall?

Make his character out to be a bad guy by a general populace despite the support of his party. Make him be an outcast of his own free will in order to conform to the perspective of a "greater good". Make him fail his people & his god in order to be truly good. Then bring him back to his feet stronger than before.

Unfortunately for you, Paladins are great with save throws.
Not only that, they're proficient in charisma (possession) and wisdom (magical influences).

If the character has no flaws (which is probably kind of bad roleplaying to be honest) there's unlikely to be any way to make them fall, and they'll be very hard to use magic to force them to do anything because of their bullshit. Unless you knock them unconscious and THEN try to make them do things, but that's still not going to cause any falling.

Your best bet is to make them question their faith, somehow.
Their god seemingly doing things against what the Paladin would hope.
You will have to ruin a god to get this Paladin to move.

...

To do that, OP could put a rakshasha or whatever they're called and make them pretend to be the paladin and attack people on the street and eat their hearts out or something batshit insane to completely ruin the Paladin's intention.

However, when the DM starts railroading it so that the Paladin's many ways of fixing the problem without doing what the DM wants doesn't work they might start to smell the smell of bullshit.

Don't "MAKE" him fall. Give him a nemesis.

someone whose methods are absolutely contrary to the paladins.
someone who's reasons for doing what they do almost exactly mirror the paladins.
make this nemesis understandable to the paladin. someone he can relate to.
then put the paladin into a situation where he either sticks to his values (but everything is destroyed including himself) or he joins his nemesis and commits a truly evil act.

in the words of the joker "all it takes is one bad day."

I'm surprise there's been no "this is bait" images yet.

That's dumb, I mean something like throwing the party into the support of a town leader who is corrupt, but torturing spies in order to keep his people safe. Making it in the best interest of the party to slaughter innocent packs of demi-humans in order to save villagers they are more akin to. Finding out a trusted colleague is diddling children and being asked to protect them from villagers.

If you're trying to 'make' your paladin fall, you're a shitty DM.

The problem here is the party might well decide they could just up and leave the village and go elsewhere.
And the more petty the reason, the more easily a party with a charismatic paladin should be able to maintain face.

Just remember to keep options open.
If he refuses to redeem, he might become an Oathbreaker.

Or, you know, take a different oath.
By the way you're laughably shit for imposing alignment restrictions in 5e when the way Oaths are presented in the book encourage that behavior while also allowing for edge cases