[N]o difference is less easily overcome than the difference of opinion about semi-abstract questions. - L. Tolstoy
The Mage power Beneficent Transformation is a good template. The spell gives you powers based on the enemy's ability. If the enemy can climb, fly or swim, the recipient of the spell gains that movement mode and speed. Or the recipient can borrow resistances or senses. In short, don't let the character completely replicate the monster's ability, but give it something reminiscent of it. If it has a power that immobilizes, give the character a +4 to grab or a +2 to attack rolls with powers that immobilize.Beneficent Transformation is a daily power, hinting at how strong this sort of ability can be, even in a limited capacity.
Don't base the character's powers solely on the enemy NPCs rather use it as a suppliment. Have it so the player takes a certain aspect from a creature which is arbitrarily assigned. like fighting a goblin blackblade will give your character additional reflex or a dragon might grant them the resistance to that dragon's element. I think that's a good balance between what your character wants and what you can do with minimal planning.
Just an idea:Reskin powers that already exist as player powers, let him get those or bonuses to stats or such. Keep a stack of index cards with things written on them. When he learns a new one over his max, make him swap out an old card for the new one. Before or after seeing the new power. Let him pick or get rid of the oldest power. He might end up with 0 at will powers and 4 encounters, but that's his problem when he uses 4 awesome attacks and then sits there for the rest of combat.That's what I would do at least. The cards make it easy to remember as well as easy to learn/forget abilities. Reskinning keeps everything balanced with other powers (since they're in reality the same). The ability to only get powers from what you fight balances out the ability to have any power from any book. Swapping out powers balances it so they don't end up with infinity powers.To learn it, have them have to be in total defence AND get hit by it (thinking the "learn" stance in FFV here). Obviously they have to survive the hit to remember it.
Google Homebrew (whatever editon your playing) DnD Classes; look for a Bluemage.
I know there was one for 4th (Im currently playing it, and having a super fun time absorbing powers) but also remember seeing a few out there when i first looked for the other editions as well
It does just what you want (with some minor tweeking); Monster casts spell; you after some rolls get that power/spell.
Google Homebrew (whatever editon your playing) DnD Classes; look for a Bluemage.I know there was one for 4th (Im currently playing it, and having a super fun time absorbing powers) but also remember seeing a few out there when i first looked for the other editions as wellIt does just what you want (with some minor tweeking); Monster casts spell; you after some rolls get that power/spell.