Tipping has gotten out of hand in America. You never tip the owner of the business. An escort operating on his own is the owner of the business. Don't tip. Reward him by hiring again for a longer period, pay for upgraded advertising, write good reviews, tell your friends, but DON'T TIP. Period.
PS: if the escort is sent by an agency, tipping is appropriate.
PPS: there have been other threads on this.