Call him and be polite. Give him the benefit of the doubt as you dial the phone, and don't take an accusatory tone to the tone of your voice. If it turns out he did indeed dump on you, don't cut him loose just yet. Tell him that you think it was a poor way to treat someone, and that if he wants you to work with him on these future gigs, you have to base that relationship on 100% trust and honest communication. I'd much rather hear "I found a guy that knows twice as many songs as you do, and sings BGVs much better" than being lied to. Just find out the truth - but don't burn your bridges, even if it turns out that the guy did lie to you.