As he's choosing the venue without discussing it with the other members of the party, couch an email along the lines of "I'm assuming the evening will be on you, as you have taken it upon yourself to arrange all of this, however if you don't want it to hit your pocket so hard, we, and the other members of the proposed party suggest a lunch at a mutually agreeable venue to all of us. Having some input obviously we will be happy to split the cost 3 ways rather than you being lumbered with a meal for 6. Speaking personally I'm more inclined to rein in my propensity for not opting for the more expensive food and wine on the menu during the day, for example, the lunchtime prix fixe usually isn't available and in any case of an evening I will be expecting a la carte, but up to you as it's very much your choice and as such you're stumping up, damn generous of you may I say though" See how that goes down!