xilo, i've been dming with one player since the advent of 3rd edition and have the following comments to offer. this edition of d&d (more than any other, imo) is balanced for 4 players who roughly represent the 4 spheres of classes (combat-oriented, skill-oriented, divine caster, and arcane caster). as such, it is very difficult to design an interesting array of encounters to challenge a party that doesn't include at least most of these class types. i've tried letting my player run 3 or 4 characters simultaneously but this significantly reduces the verismilitude of roleplaying, as you might imagine.
another option is to let the player run only 1 character and then have the dm run several npc's to round out the party. this becomes an overwhelming task for the dm, i find, as s/he is already burdened with playing all other npc's, maintaining continuity, providing evocative description, etc.
of the two, i'd lean towards the former option. though roleplaying is greatly reduced to rollplaying in this option, there is more similarity in the breakdown of duties in such a game (the player rolls all the dice and makes decisions for the heroes, the dm does so for the villains and monsters as well as designing the storyline).
another option i'm considering is letting the player create several characters, running each of them on solo adventures to develop personality, etc. and running all of the characters for occasional more challenging sessions.
drop a line and let us know how things work out!