How to explain to AD&D player that items that increase Armor Class in DnD5e are overpowered?

Context: I have recently started a campaign with a new player that comes from AD&D background. He wishes to convert and import his character to the campaign. We do that once, I help him with the conversion and checked his stuff, he got 2 magical items lvl3 a bit much but I didn’t mind they were not game-breaking (ring of immunity to poison and mobility boots(longer jumps and a bit of speed up)? His character died, He asked to bring another character this time I let him do, I ask him for his stuff and he says that he has a magical sword and a giant strength belt (lvl6) this is IMO very strong, but again not game-breaking I tweaked a bit the encounter and it went fine.

The thing is that I give him a full plate armour and then he says that his armour class was 23. I was surprised because Max AC without magic is 20 – 21 (with defence combat style (which he has)). So I asked how His sword is :

The Items magical sword +2 attack bonus, magical damage, +2 fire damage, +2 AC.

IMO At this level having a CA of 23 (25 with "shield of faith" he plays a paladin) is game-breaking most monster exception the strongest get attack bonus of +4 +6, Meaning that he will only get hit on crit by most monsters, or that I have to send powerful monsters (that is maybe not fine for other players). The first question is that True? is this AC game-breaking at level 6?

I started to talk about this. and it went bad. He says that it is only 8 % increase of AC and did do much and that he gains his sword on a very deadly quest etc…. Maybe it is a good computation in AD&D setting but in 5e I feel it is not. Does it exist any rational arguments about this?

For now, I come up with the following arguments:

  • AC doesn’t scale the same, and 21 is already the maximum you should expect to be. A venerable red dragon AC is 22.
  • AC increase is extremely costly, with non-magical means they are only two ways: combat style and feats (heavier armour type or the Medium Armor Master feats (+3 dex))

Does there are other arguments? Or maybe I wrong?