dnsmasq is the software that controls the dns forwarding aspects of pi-hole.
what you want to do is go to /etc/dnsmasq.d/ and add a new configuration file (editing the others will be temporary as they will be rewritten by pihole). name the new file 05-philippeake.conf, to indicate that its your custom config file.
in there, add the following:
strict-order
From the dnsmasq manual page: "By default, dnsmasq will send queries to any of the upstream servers it knows about and tries to favour servers that are known to be up. Setting this flag forces dnsmasq to try each query with each server strictly in the order they appear."
then use the command "pihole restartdns"