If the gig is booked using just your face and contact, then going with a sub for the night will not even be noticed. Ditto playing it as a 3 person group.
You play for cheap and free so really it's a charity social club band and deserves to be lower priority in band members' lives than paid work and family time. If you need the guy to be there, pay him, then he can prioritize. (But even with $, travel time with family/friends is important and can be worth more than a gig night's salary.) You have very little leverage here.
Are you performing this guy's (2nd guitar/backing vox) original music and are wondering why he, the star composer, is backing out? Justified.
Doing a bar gig with standard material for free, and are upset that instead he wants to do a weekend vacation with people important to him? Not justified.
For those who say "fire him", remember that you have to be in a paid working job to be fired. This is not that.