Please ask this on the pfSense forums. While we could make some guesses of how pfSense configures its dhcpd(8) our guesses about the integration with the other components of the pfSense system could be miles off.
You can set hostname via DHCP using DHCP option 12, if client requested it. It should be possible (I never used it personally) to request hostname with
in the /etc/dhclient.conf file. There is hostname option on the Static lease configuration for the DHCP server in the pfSense GUI, however documentation on what it does is lacking. You have to experiment yourself or ask on the pfSense forum as kpa already said.