For bios based computers you would use undionly.kpxe as the default bootloader. For uefi based systems the boot file is ipxe.efi. The next server or boot server value should be the IP address of your fog server.
I don’t have a clue, but could figure it out how to setup a dhcp forwarder on the Aruba/Procurve switch. But I would have to say, if you have a single dhcp server today AND you have vlans in place then you should have a dhcp-helper/dhcp-relay service already installed on your vlan/subnet router. The only reason why you would need to change this service is if you are running dnsmasq service on your fog server (something you would intentionally turn on and configure). If you are not using dnsmasq then changing the dhcp-relay service is not needed.
Can you explain a bit more about what issue you are having?