If he has engaged their service he has, implicitly agreed to paying, whether he signed or not.
What that price is at this stage is what the issue is about.
What service is free in this world? Are yours?
Is this agent an estate agent?
A renter doesn't engage the services of an estate agent, they work on behalf of the property owner. If the estate agent informs a prospective renter (before he signs a lease) that there is an additional fee which hadn't been mentioned before, the prospective renter is well within his rights to walk away without paying anything to the agent.