Pokemon with hold items in the anime tend to show in some way that they're holding them, like Misty's Poliwhirl wearing a King's Rock on its head before it evolved.
I doubt it's a Light Ball, otherwise Pikachu would be holding one.
Also, Light Ball raises both Pikachu's Attack and Special Attack as of Gen 4 (I think?). And Iron Tail is a physical move, not a special move. :P
Either way, I think his Pikachu is just strong, as he has been traveling with Ash for 5 generations going on 6.
Yeah, held Items have usually been seen in some form in the anime, either the Item itself, or as headgear (like King's Rock), or as a pendant of some sort (like Mystic Water). My theory (derived from someone who suggested it a long time ago on these PokeCommunity Forums, I think) is that Pikachu just
swallowed a Light Ball at some point when it was little, but the Light Ball still "counts" as its held Item (except that other Pokemon can't take it away and stuff XD). Although I'm worried about the day when Ash actually bothers to give his Pikachu a
visibly held item when he finally realizes the usefulness of held Items in battle... >.<
I'm not sure what Arceus did with his plates in the Arceus-based movie, though...did he actually "hold" them or did they just levitate in front of him and stuff...? Or were they just contained "within" him during battle...? If they were contained "within" Arceus, that could help support the theory that ingested/"internal" Items still produce an effect on the wielding Pokemon in question...
Alternately, one could suggest that Ash's Pikachu swallowed Eviolite (explaining its high tendency of survival against a lot of opposing attacks) and just gets insanely lucky with Critical Hits... And Ash's Pikachu could also have flawless (+31) individual values/IVs in all of its stats, so there's that...
Brock's dead just helped Ash train Pikachu to let it hold more electricity at once. Pikachu was also struck by a massive bolt of lightning in the first episode.
Hmm...other than Pokemon-Amie helping with "avoiding attacks", "Critically Hitting more often", and "not fainting", there's not really an in-game correspondent to the training from Brock's dad and the EP001 lightning bolt... I'll just chalk those up to anime-only occurrences, then. :P