Also I'm not opposed to throwing books into my game sessions that "reveal" studies or whatever that my players can use to boost their knowledge through study. This works for magic users who find out some revelation about spells or for martial types with pictures and instructions on particular moves. I've also thrown in books that have studies of different species that if studied for a while give boosts to different knowledge checks or similar items. So they can go beyond just what they get from leveling if they actually participate in the roleplaying aspect of the game.
So why doesn't it work for the martial character studying a book of arcane lore at a basic level, or a spellcaster reading a basic martial training book?
It seems like you're taking a slightly different approach, giving players a freebie if they do some extra role playing. The sorcerer gets all his usual abilities, plus a bonus feat (warhammer proficiency), because he worked with the fighter every evening and practiced back in town. Can he now work with the Wizard to learn how to Extend his spells or scribe scrolls and get that feat for free? Can the Rogue teach him how to Sneak Attack? Why does he get the Warhammer proficiency but not proficiency in some other martial weapons? Why not a free fighter level gestalted with his Sorcerer level? As payback, can the Sorcerer teach the Fighter how to cast, say, True Strike or Haste?
It's a more free form approach to gaining character abilities. Whether that's good or bad depends entirely on whether the group is enjoying it, but much less consistent/reliable/predictable than getting the abilities your new level provides - no more and no less.
It could have been as simple as looking at the fighter and saying "hey does your character mind training him in some basic combat abilities and weapon use? You will cool. Hey sorcerer during down time you're working with the fighter on basic combat stuff but you're not going to reach his level of precision that quick so you end up as a barbarian." That allows them to play off of each other during game play, gives reasoning for what's happening and gives the player what he wants for barely more brain power than just having him write barbarian on his sheet when they level. To me that's a better way of getting things done instead of just having new things pop up just because.
So, if working with the fighter is enough to gain the Sorcerer access to the Barbarian class (not sure how he gained rage, enhanced movement and all those extra skills - all that training must still be in the background somehow), does that mean that the Fighter could choose to take a Barbarian level with no extra training, or does it only work when his fighter skills are taught to non-fighters?
To the thread title question - yes, he has to earn it. He earns it by gaining sufficient xp to gain a level. That earns him the right to go up a level.
srd said:
First, select your new class level.
Instead of gaining the abilities granted by the next level in your character's current class, he can instead gain the 1st-level abilities of a new class, adding all of those abilities to his existing ones. This is known as “multiclassing.”
That's how he "earns" multiclassing. The example is a fighter who takes a wizard level, which seems no different from a sorcerer taking a barbarian level.