If you like going solo, just disregard this first idea... but, have you thought about moving your game to online. I have a friend that was playing solo and they were getting burned out on just the two of them, so they moved to openRPG and skype, so that more people could get in on the action. Next thing you know, there were 7 in the group.
Next, do you play more sandbox style or scripted? If you have a premade city, the old JG citystates come to mind, then you could let the player wonder around and kind of "choose" what he wants to do. This is a more advanced option where the DM has to be ready for just about anything. A classical dungeon crawl kind of assumes that the 4 archetypes will be represented in some way or other, so that would be kind of difficult, IMO, to pull off.
Other adventures would depend strongly on class, but I do like the option someone made of giving the leadership feat for free, so he can have a cohort at least. NWN was like this, and it was quite fun for a computer game. This would at least give you two of the four archetypes, and depending on your classes, you could perhaps summon cannon fodder.
Rogues would seem to be the easiest class to run solo. They could be part of a thieves guild where they are sent out on assignments and given resources (like muscle) when they need it. Other classes could be part of a guild as well, you will just have to be creative.
I grew up in a small rural town, so I played a lot of times with just one and two players and I DM'd. I liked the sandbox approach where I would try and let the party pick out what they wanted to do, so as not to get the feel that I was railroading them all the time. This was great for more experienced players, but people new to the game need more structure, for them I would use the "assignment" method and tailor adventures to them, and let them move up in the guild as they moved up in level.
Best of luck to your game,
Athos