Good advice already. Check out the Wu Jen and Shugenja spell lists for some cool ideas, since those are very elementalist-spellcasters. Consider the usual Fire Resistance/Fire Subtype/Fire elemental slaves, but also consider giving him some weaker Cold Resistance (to signify how warm his body is). Consider giving him bonuses in lighted conditions, and harsher penalties in dark conditions -- encourage him to carry around open flame as much as possible.

Don't neglect environmental conditions, either...a bonus to resist extreme heat, and extreme cold, could probably be in order. And the usual "fire-style" familiar would be a good recommendation...I believe one of the Improved Familiar lists has elementals in it, so those could work. Fire breath, phoenixes, fire-based dragons, salamanders, the list goes on and on for creatures who should consider him kindred. Giving him flaming weapons is probably a no-brainer, but consider giving him "flaming" or "flaming burst" spells, where his magic deals a bit of extra fire damage. And make sure to give him something effective against those who resist fire (though I'd probably consider more of a charm-style effect for that, rather than straight damage).
Here's some new, not-fully-fleshed-out ideas that could be fun:
Combustion Spell: People catch on fire when they fail a save. Applies to all spells -- magic missiles light people on fire, now.
---> NEW SPELL:
Ignite: Simply lights stuff on fire.
---> NEW SPELL:
Fanned Fire: If something is on fire, this can cause it to leap to a new target, as well.
Explosive Spell: Those who fail a save are knocked down/knocked out to the edge of the area.
Heat-Seeker: Evasion and Improved Evasion don't simply halve or negate damage against this spell. Instead, Evasion grants you a +2 to bonus to the save, Improved Evasion +4.
--->
Heat-Vision: Concealment miss chances are reduced by 10% for you.
Fire Spikes: When he takes damage, fire sprays out and damages the attacker. If he is killed, he blows up all pretty like...destroying his body, but probably a lot more, too...
--->
Melting: Things that attack him may melt or steam away or catch on fire...perhaps a DR of a certain value is best for this effect, perhaps something like the Rust Monster's attack.
Rocket: A move that lights yourself on fire for a charge attack, dealing additional fire damage (+1d6, maybe) on a charge. Consider combining it with a fireball spell...when you cast
fireball, instead of shooting a bead of flame, you BECOME a bead of flame, and travel the distance it is cast, and burst out when you reach your destination.
Ghost Fire: Doesn't suffer a miss chance on incorporeal creatures; deals more damage to undead.