If it is the first website for this IP, he can access the site with the IP.
If not, the only solution might be that you add temporarily a co-domain to that site
that already points to this IP. E.g. make some temp domains like temp1.yourcompany.com ..... temp10.yourcompany.com that points your server. If you get a new customer he can
access his site with this domain until his real domain is registered.