[Agrajag]

Are his Undead units etheral or not?
If they arent, then he must be ashen empire Ermor, which means he can reanimate with no need for corpses OR population.
Sieging him might work, but he still gets his hordes of freespawns.
He does get less if his dominion is weaker, and even less if he has no castle.
If you intend to sieg his capital, you'll need to constantly increase security as he will constantly recieve new units and (probably) will use them to break the siege.
Also, supplies arent a problem for him, so his castle army can grow without worry.
For income you can wish or use "Gift of nature's bounty" if you are capable of casting any of these spells (the Gift is an excellent income spell, especially with all the provinces I assume you have.
Another good idea would be researching all the mass undead destruction spells, Grind Bones iirc is very useful against undead (if you can find someone to cast it).
Oh, and taking his capital will weaken him alot because it will take away his Sepulcher, his best death gem income site (and we all know that AE and SG Ermor live and die by death gems, not sure about BE though).