Ok so at 20th level he'll have 13th level(7th level spells) arcane casting and 15th level(8th level) divine casting? Then at 21st level he receives the Lich Template?
Seems reasonably balanced. I can see why someone would want to take this class.
I love this class a lot actually, just one thing that would be cool. What if he could choose different undead forms to turn into at the last level instead of just a lich? I mean vampires are awesome too.