Vocal lesson?
Better preamps?

No program will fix your voice, everything happens outside the box, the programs are there for editing.

Nuendo is a good one?

Nuendo is a good program but your program will not improve your recordings.

If you want your voice to sound better then you need to improve your voice. Software can't do that.

You could buy Autotune and use that but it's not going to help anyone.

how do you get the sound from the Mackie into the computer? Are you using the computer's sound card? IF so, that's not really all that good.

Think of software as cassette tape....there really wan't much of a difference in sound from one brand of tape to the next...same goes for DAW software.

The problem could be in any one of the devices you mention.
Maybe the microphone doesn't work with the voice...or it's being used improperly.
Maybe the mixer isn't set right or adding an aspect to the sound that is undesirable.
Maybe the Midiverb is crap.

My first step would be to remove the midiverb from the chain...so it's mic->mixer->whatever audio interface you are using->DAW.

If he has an Onyx 1640 he should be going in via the Firewire option. If not then he needs an interface me thinks. And start without the midiverb until you get the signal flow correct.