Just have him learn more spells from scrolls. If you're playing a mage and have sufficient INT & WIS, and you ask Dak'kon to teach you the Way of Zerthimon, you can teach him (and yourself) some more of his unique spells. See
NPC Conversations - Dak'kon for more details.
When I'm not playing a mage, what I do is buy all the WIS tattoos and save up the Cranium Rat charms, switch to mage, apply the tattoos and use the charms to boost INT & WIS, so I can unlock the final circles to power up Dak'kon.