It doesn't matter so much for Holy Paladins, being more of interest to Ret and Prot Paladins but here is a break-down of how it works, with Holy-specific information beside it.
Holy Power is somewhat similar to Rogue combo points in that they build up over time if certain abilities are used, to a maximum of three charges. For Holy Paladins, these points are generated by Holy Shock and, if you have the talent Tower of Radiance, direct healing the target of your Beacon of Light.
You can use these charges to enable or boost some skills. Holy Paladins would be most interested in Word of Glory, an instant heal that heals for some total based off how many Holy Power charges you currently have.
This mechanic is a lot more useful for Ret Paladins, who I think it is mainly aimed at. They can charge it using Crusader Strike (for a guaranteed change) or, if they have the Divine Purpose talent (which they should), have a 40% chance of getting charges from just about any Ret skill, i.e. Judgement, Exorcism, Templar's Verdict, Divine Storm, Inquisition, Holy Wrath and Hammer of Wrath. They can then use it to charge their Divine Storm (AoE damage), Templar's Verdict (single target damage) or Inquisition (holy damage boost), or Word of Glory if they want/need a quick heal.
Prot gains Holy Power from Crusader Strike and Hammer of the Righteous, and mainly use their charges on Shield of the Righteous, though Word of Glory is obviously also an option for them as well for a quick self-heal.
I'd suggest letting him play at his own pace (unless he's an experienced MMORPG player starting WoW for a specific end-game purpose such as raiding / PVP). In particular if he's new to the genre, that first time exploring the world is such an amazing time, it would be a shame to be boosted through it.
Following the 'Shattering' the starter zones have had an overhaul and are fun to play through and also give a better step by step introduction to game mechanics and how to play your class.
Also the levelling speed now is pretty crazy - people in my guild are making jokes about being level 30 by lunchtime! It's not quite that fast, but if he gets into it he'll be 30 before he knows it.
Best Answer
Yes, both the Sigils of Power and Wisdom are obtainable from LFR, as is the follow-up quest item and all of the Sha-Touched Weaponry that tie into the Legendary quest. This is by design, as Blizzard feels that all people who raid (at the LFR level or better) should be able to see the quest line, as well as increasing the "value" of said quest lines (as everyone can do them, not one specific class a la Fangs of the Father).
Note that when they do drop, all raiders on that step of the quest get the Sigil - there is no competition for it, as in past legendaries. This is true on all raiding difficulties, as well.