Three things you can, and should do:
- Admit your mistake. Tell the player, "hey, I misunderstood the shield rule here, you can weild two, the second just doesn't do you any good." It's important to own up to this asap.
- Let him rebuild his character from the ground up. You've clearly found out his character concept no longer works the way he wants to. Offer him the opportunity to rebuild the character completely with level appropriate gear etc.
Offer him the Testudo Shield fighter as an option. If he loves the concept enough to keep it, even without the AC bonus, MCing fighter and taking the Snapping testudo paragon path would be the best option. This is kind of a crummy option for a PP, but it's a cool concept.
If you decide to allow this, change the bonus type on the second shield. My recommendation would be to allow him to take the homebrew feat "two shield fighting" that allows him to use the shield bonus of the second shield as a feat bonus to AC (or AC and Ref, and additional defenses if he spends the feats on them). This is pretty fair, it's a bit high in heroic, on par in paragon, and probably a retrain candidate in epic. Check his final AC, but it should be about the same as a pally in full plate with a heavy shield. Make sure the penalties stack and offer a feat to remove one at heroic (he can take a feat to remove both in paragon).
All this to say, own up to your mistake, give him an opportunity to fix it. But as a note, if you have DDI, build your characters in the builder, it does all the math for you and it will catch a lot of these bonus stacking issues your pointing to.
Mostly though, just talk to your player, explain your mistake and see what you two can work out. Communication is good and your both learning the game, keep it honest, and keep talking.
Ah yes, the effects of age, I could tell you a lot about that. But I’ll try to keep focused on in-game effects.
“I’ll regret this later”
Penalties to abilities that simply make the character weaker (like those found in older versions) are frustrating for a player, and they are bland, and don't really evoke the feel of old age. These rules simply never provided any fun, or much depth. It’s no wonder they didn't come forward to 5e.
Effects that limit the endurance a character has, that come into effect after some initial exertion, evoke the effects of age more poignantly, while letting the character still “relive past glories” in brief but glorious combat.
So very tired
The rules for exhaustion in the DMG can be utilized to simulate the fatigue of advanced age, and they come pre-play-balanced for you. A venerable character (depending on age, at the discretion of the DM) may gain a level of exhaustion from a single combat, and require a short or long rest to recover from it.
I just can’t seem to rest like I used to
Speaking of rest, an older character needs more. Simplest thing would be to double the amount of time needed to attain the benefits of the rest, but it doesn't add much color to the game, if the players are simply obliged to say “OK, then we rest for 16 hours.”
I find it’s better to “nerf” the effects of the rest, granting back fewer hit dice, hit points - or even fewer spell slots, if mental ability has been affected. That allows the oldster(s) to “try to keep up” while providing a nagging reminder they are really too old for this sort of thing.
(If you ever played 4E, this was like a non-heroic NPC taking a rest. Unlike the PC’s, the NPC would not wake up in the morning fully healed of all wounds.)
Another nice surprise
I would also suggest that these or any effects of old age should come as a surprise (especially for the prematurely aged). They always do.
My (game) experience with old age effects
I’ve used these rules for adjunct NPCs - I haven’t prematurely aged a PC. The players thought they were fair and interesting rules. It accentuated the power of the (young) PC’s, without making the NPC useless. The oldster slept/rested while the characters did ancillary stuff. It might be different for a PC. I suspect a player won't be overjoyed with having to “take it easy” but it's similar to other “curses.”
Since initially answering this, I spoke with one of my players about the age rules we used. She commented the rules made her feel protective of the NPC.
Best Answer
Alignment is a description of how you have acted in the past, NOT a restriction on how you act in the future
There can be no penalties because alignment is not a straitjacket. This was one thing that 4e absolutely got 100% right (though I prefer the 9 alignments to the 5).
Playing out of alignment may anger others who share your alignment, may draw the attention of people who prefer the alignment that you’re uncharacteristically acting like (e.g. cause Evil creatures to try to corrupt you more), and so on, but these are all roleplaying issues, and that is all they should ever be.
The Dungeons & Dragons alignment system is simplistic and binary (well, nonary, I suppose). It can be a little useful as a shorthand for what side you are on, but trying to use it as anything more is a mistake that leads almost solely to arguments.
There’s a quote I like particularly well here: