Rogue/Wizard/Unseen Seer/Arcane Trickster feels like a pretty viable way to go (or switch in Sorc). Wizardly casting is so good that you want to be as much of a Wizard as possible, at which point you're adding some skills and a Rogue-ish flavour but you're very much a caster. Beguiler is another good option here (either straight Beguiler or that with Beguiler instead of Wizard) as it already has a Rogue-ish flavour right off the bat.
From the little we know of this guy, he strikes me as more of a martial type. If you wanted to mix in casting, Daggerspell Mage from Complete Arcane is actually not bad at all. Assassin is also OK; it's oft-overlooked because it's just kind of lacklustre (especially when compared with Wizard into Unseen Seer or Arcane Trickster) but it has a lot of what Rogue has plus some (admittedly very minor) spellcasting, and with the Spell Compendium they get some decent choices.
From the little we know of this guy, he strikes me as more of a martial type. If you wanted to mix in casting, Daggerspell Mage from Complete Arcane is actually not bad at all. Assassin is also OK; it's oft-overlooked because it's just kind of lacklustre (especially when compared with Wizard into Unseen Seer or Arcane Trickster) but it has a lot of what Rogue has plus some (admittedly very minor) spellcasting, and with the Spell Compendium they get some decent choices.