When he gets a skill advance he can choose to randomly select a new prayer.
From Campaigns, Advance Rolls, Heroes, "New Skill. Select one of the Skill tables available to the Hero and pick a skill. If he is a wizard he may choose to randomly generate a new spell instead of a skill. See the Magic section."
From Magic, Allocated Spells, "Each wizard starts with one randomly determined spell, but may gain more. Roll a D6 and consult the appropriate chart. If you get the same spell twice, roll again or lower the spell’s difficulty by 1."
A Tome of Magic is a nonstandard way to gain spells.