Give them your stats, let them know how many guys you've got coming in. You need to make a good impression and prove that it will be worth his time and stress to have you guys play there. If you have a good amount of guys and he'll be making a little on the field fees, then awesome.
He may also be able to retail BB's and Gas?
I know some insurance companies are very against airsoft - even though the players are generally more mature and you have a whole lot less of that "12 year old taking off his mask as soon as he's hit" sort of deal! But nonetheless, it comes down to the individual insurance company and what their policy is.
I've worked w/a couple different paintball fields, and even when we had an awesome working and business relationship, what the insurance company required was always the bottom line. We were required to have full face masks (blah) and barrel plugs...so.
But get organized, write it all up, have your group all sign it - let him know you're serious and dedicated. Offer free help working on the field? Cleaning up? Etc.
And good luck w/his insurance!