It can be done. It can be done well.
The biggest things I can think of:
0) Make everything cinematic. It's a one-man show; you have the time to describe actions, locations, appearances, and effects in detail that make the entire story ineteresting and cinematic. Make sure to encourage his descriptions of events, actions, and so forth.
1) Losing a fight won't usually get him killed: like any solo hero (pulp novels are a great example, as is Indiana Jones) he usually gets knocked out for interrogation, thrown into a death trap, or otherwise left with a way to make up for that previous failure.
Of course, another failure will end his story. You can't be too generous, or it stops being interesting.
2) Lots, and lots (and lots) of non-combat encounters. Role play, skill-based challenges, and other stuff that moves the story and accomplishes goals without serious risk of killing the character.
3) Center the story entirely around his choices. Trouble may come looking for him but the story is about the PC, not the world he's in.
This is true for any campaign but especially true for a solo campaign.
4) Excessive items. Give him a more items than usual, allowing him to have really good gear for his level. It helps a lot.
Healing potions, items that give options (Thunderstones, Tanglefoot bags, etc), items that provide isight and warning (Gem of True Sight, Alarm chimes, etc), and items that give him the ability to reroll a bad die roll from time to time should really increase survivability without making him broken.
5) Helpful NPCs. Hirelings to provide needed skills; merchants to provide rituals, items, or spells that he can't provide for himself (for whatever reason).
6) Lots of good advice appropriate to the character's experiences and mental statistics. Make sure to describe things in ways that are likely to give him useful insights and helpful guidance.
If he's really stuck, make a few suggestions that
only use the knowledge his character has. This can still lead to badness but that's his fault for not looking into the highly suspicious activities that gave the big clue of victory.
7) Say Yes. If an idea is good, makes sense, or would be fun to watch then let him do it. And set the DC appropriate to how much you want it to happen and less about how hard it should be.
8) I'd recommend some sort of super-action point mechanic. One that really lets the cinema shine through and holds the unfun story-inappropriate random death at bay. Something like
Conviction would be good.
Good luck.