I have a player who is a necromancer.  It is a bit underwhelming as he does not want to build an army of undead.  Instead he would rather have a consistent undead that would grow in power as he does.  He is even for using greater spells to be cast daily to maintain the creature etc.  I am trying to work out how to make this fair and reasonable in terms of utility and damage.  Has anyone done something like this or have any ideas how to scale up undead to be followers?

There was a post on this exact topic on another forum a few days ago.

Essentially the answer is that it already exists in the form of summon undead, but as with any spell that summons a powerful creature, it has to have a limited duration.

Spells that summon creatures simply don't summon powerful creatures and then maintain them. There are class features that can do that. I suggest you look at the Homonculus for Artificers and homebrew something around that, but understanding that the type of undead minions your necromancer is supposed to have should never be more powerful than those granted by temporary conjuring spells. summon fiend is about as good as it ever gets.