All good advice so far.
More than anything, try to be cognizant of the differences between solo play and make sure to focus on its strengths and away from its weaknesses.
Think about all the situations wherein as a GM you had to be wary of one player dominating the game and how you had to rein that in. When a PC dominated roleplaying conversations with an NPC. When the Rogue sneaked off for a lengthy scouting mission by himself. When the Cleric was having a religious epiphany that bore lengthy discussion concerning the philosophical ramifications of that event. All these sorts of situations lend themselves very well to solo play. Use that.
Make sure you look at that PC's character sheet and design situations that he will thrive in. If he's got a great Climb skill then put in plenty of cliffs and walls to overcome. If Diplomacy is his thing then make sure he spends a decent chunk of time in courtly intrigue. If he has Burning Hands as a spell, make sure that a mass of weak creatures attacks him at some point (maybe a weak Swarm of some kind).
When it comes to fighting, err on the side of caution. Use monsters that will flee when heavily damaged rather than fight to the bitter end. Another good option is creatures that only attack a "threat", like maybe Skeletons. If the PC goes down then they'll stop attacking, giving them a chance to stabilize and awake later and attemp to sneak away. Watch out for weapons that have high crit multipliers. I saw a PC die to the very first attack roll of the entire campaign when a Great Axe critted him. Start off with weak foes with simple weapons (Kobolds with clubs).
Lastly remember that you don't have to concern yourself with party balance. If you gave out a +2 Sword that healed the wielder 1/4 of the HP damage that it inflicted to a party of 4 first level characters then one person would suddenly be much more powerful than the rest. But in a solo game this is not a problem so long as it keeps them in the power level that you're comfortable with at the time. In a solo game I ran for my wife she played a Wizard. As a "graduation present", her master gave her a staff that summoned a Lantern Archon for a few rounds each day. This gave her the option to bring in a companion in a tough fight who could bolster her with extra hit points in the form of Aid.
Good luck, have fun and let us know how it comes out.