Man, I've never played for any band or anything, but I understand that feeling of intimidation.
But I would strongly suggest that U condition your mind not to feel intimidated, but take it as an opportunity for you to ask what that guy was playing..the outcome of this:
i) You will learn new stuff there & then
;or
ii) The dude could either play in any key & adapt to any situation, but still not be able to explain what he's doing because he maybe lacks the theory--now it's your opportunity to offer to teach him the theory & he in turn can show you what he's playing & his voicings etc.; or
iii) As T-Block said, the guy could just be transposing etc, now you can ask him to show you what he's playing in that key & challenge him to learn it in all the keys/ ask him to show you in another key. Now you have created an opportunity to help him truly advance in his playing & you've created a line for you guys to shed together.
I must also congratulate U 4 initiating this band--it's by no means an easy task.
Since you thought the guy's playing was so superb, & if you feel that that's the sound you want your band to adopt, then invite the guy to be a member of the band ( & don't be intimidated by his presence
).
If he's a true musician/ mature guy, he'll find it as a responsibility to help you be at the same level that he wants the band to play in.
You should also tell him that you are still a beginner & will need his guidance in many areas (that's if he really knows what he's doing & not just showing off!).
With that set, you could expand you band, say with you taking on the synths or organs (or whatever sound you want to incorporate since there'll now be 2 keys players.
Better still, since it seems like he is so good, you could have him play 2 boards 4 diff sounds & U play the 3rd 1
Well, that's just my 1 cent's worth & hope it's value can appreciate .
Best wishes with your playing