CapnZapp
Legend
Hit him where his great stats don't matter. His allies. His family. His plans and items.So I've never had a problem with heavy armor. Sure, it's high AC, but it normally means at least a subpar Dex save. Problem is, is the Paladin.
Meet Mr. Tank. He's got a +4 Charisma modifier, +2 plate mail, and a shield. Nothing I throw at him hits(/affects) him (90% of the time). Where a weakness in saving throws would be, it's covered by his aura. Where a weakness in output would be, it's covered by his smiting and/or channel divinity.
Asides from throwing Magic Missiles at him, how can I stop this juggernaut from being such a... juggernaut?
If your player enjoys being nigh-invulnerable, let him. He did pick the right class for that concept after all.
Don't get bothered about not being able to touch him. You can't. Not without hosing his build or by throwing so dangerous monsters at him that you critically endanger the other members of the party.
Your best bet is to just wait. Most players will eventually become bored when they never feel any danger. Perhaps he'll play a clothie with 8 CON the next time, just for funsies.