The section on creating higher level characters (pg 143 of the DMG) suggests that powers, and by associaiton spells, should be limited to one known/prepared per available level. i.e. a 15th level character has a 15th, 9th, and 5th level slot any of which might be used to hold a lower level power, but non of which can be used to hold a higher level power. So you can't have a 15th and two 9th level spells prepareed, but you could have a 5th and two 9th level spells with one 9th level spell in the 15th level slot.
To make the whole thing easier, I'm tempted to give the Wizard 2 (or three with Expendad Spellbook) spells of each level as that level becomes available. Those automatic spells generally stay in the book, so a 15th level wizard would have 2 spells each of 1st, 5th, 9th and 15th level, but would still only have a 15th, 9th and 5th level slot. Theoretically he has lots of extra powers known, but those first level powers are rarely going to be prepared in the higher level slots since higher level powers are generally more potent.
If the wizard chooses, he can retrain lower level spells to add additional higher level spells to his book, for example dropping a 1st level spell to add another 15th level spell, leaving him with one 1st level and three 15th level spells of which he can still only prepare one per day.
To make the whole thing easier, I'm tempted to give the Wizard 2 (or three with Expendad Spellbook) spells of each level as that level becomes available. Those automatic spells generally stay in the book, so a 15th level wizard would have 2 spells each of 1st, 5th, 9th and 15th level, but would still only have a 15th, 9th and 5th level slot. Theoretically he has lots of extra powers known, but those first level powers are rarely going to be prepared in the higher level slots since higher level powers are generally more potent.
If the wizard chooses, he can retrain lower level spells to add additional higher level spells to his book, for example dropping a 1st level spell to add another 15th level spell, leaving him with one 1st level and three 15th level spells of which he can still only prepare one per day.