I ran a solo campaign for a friend that went from level 1 to level 13.
He played a (3.5) dwarf ranger and used a waraxe-shield combination. To this day, he says it was his most enjoyable DnD experience.
The plus side of solo campaigns is that a character can be really, really fleshed out as they level. The focus on one person can really capture subtle changes in attitude, alignment, etc. that are tough to eke out in larger games.
Oh! And combat moves much, much faster and more fluidly.
The downside (as I saw it) was that I absolutely had to intermittently toss in a few NPCs to assist him or for him to lead and direct.
If I were you, I'd give the player a sidekick early on, preferably one with NPCish stats (that is, low) and even a level or two behind the player. Keeping the NPC alive and directing them in and out of combat sort of makes it into a more tactical game for the player and maintains the semblance of 'soloing' while also not derailing the campaign's longevity.
But! Don't just dole out healers! Giving a warrior a skill-user also makes things a bit on the easy side sometimes. And I remember having to auto-confirm a few things that would have been impossible for the player to get through without dedicated mages too.
There were quite a few NPC allies that weren't his friends, but toward the end of that campaign the player had a group of perhaps half a dozen loyal followers, all of them a few levels lower than him and with very specific foci.
You kind of have to think more cinematically with just one player, too. Let them do things that skill checks don't always cover.
