His mix came back and everything sounded lifeless. Not really sure how to explain it, bit of a newb at engineering. At first we thought, maybe our gear sucked or we tracked using the wrong mics, or did a rubbish performance, but the other engineer(s) made the tracks sound really good.
Here's a bit of unfortunate truth; back when I was a lowly sales person (a long time a go, in a galaxy far, far away), all we had to do to make your Average Joe customer buy a piece of gear, whether it was a loudspeaker or an equalizer, was to make it sound louder than the competition. Just boost the output gain on the EQ by a few dB, even while leaving all the bands flat, and the result will sound "more lively" to the average customer. Want to sell a pair of speakers or studio monitors? just make sure you A/B compare the ones you want to sell against speakers that are less efficient (put out less dBSPL for the same amount of wattage.) Regardles sof how they sound - unless they are just absolute crap - the untrained ear will go for the more efficient ones eight times out of ten, regardless of the actual quality and frequency response of the sound itself.
This is going to be reeeeal hard to get across without sounding like an eliteist insult (now I know what Obama is going through

). Please don't take this the wrong way, but are you guys really the ones who should be judging? I mean, if you can't tell the difference between the quality of a mix and the quality of performance or quality of gear, then you probably don't have the ears to tell what is actually a quality mix to begin with. Sure his mix may be "lifeless" to your ears compared to the others, but be honest; if you can't tell whether it's for any of the reasons you listed above, then you probably won't be able to tell it's simply because he has only created a mix alike he's supposed to, and not thrown that mix against a mastering brick wall the way a more inexperienced or devious "engineer" would automatically do to make it sound "k3wl" to the untrained ear.
Maybe his mix is indeed worse, and you are absoluetly right. Maybe his mix is fine and you guys are just gettng blindsided by premature extreme limiting on the part of the others. If you can't tell for sure yourself which is the case, then maybe you should try getting a corroberating or second opinion from more experienced ears.
And yeah, communicate with the guy now and make sure you are both on the same page. And the next time you have something like this done, communicate with the guy and *set expectations* on both sides BEFORE any work is done.
G.