If we assume that Kel'Thuzad managed to regain his corporeal form since his defeat in Naxxramas, why doesn't Bolvar send death knights to go find him. Since Bolvar can "feel" the undead, maybe he's also able to "feel" where Kel'thuzad's phylactery is currently hidden. Also even if he didn't regain his form, maybe Bolvar can fasten it somehow if phylactery is brought to him.
Kel'thuzad is probably one of the most knowledgeable being in Warcraft universe when it comes to Legion:
- We see from War3 that he knows about Legion when he explains to Arthas about them
- He was the one that indirectly started the Third War and successfully summoned Archimonde on Azeroth
- He read the book of Medivh and probably gained more knowledge about them
Kel'thuzad was fiercely loyal to Lich King/Arthas and it makes sense for Bolvar to try to exploit this and enlist another powerful lieutenant into his army.
We know that his servitude to Legion was just a ruse, and that he hates them as much as we do. I mean since Legion invaded again it makes sense to enlist his help if he's willing to join Scourge/Ebon Blade again.