if you get rid of his ability to attack then you can have an additional 1.
if you get rid of his ability to move then you can have an additional 4.
otherwise, as Lifee_72 said, you can use a spellbook to give your hero an additional 10 spells (+11 spells in the spellbook, -1 for having a spellbook).
also, you can put a spellbook inside of a spellbook if you are really feeling brave.
keep in mind though: a hero can only have up to 5 HERO abilities total, if you want to have more than 5, then abilities will have to be UNIT abilities.