If the DHCP server is on the same subnet, there is no need to do any "forwarding" at all -- it just needs to be online and enabled, and it should just work. I mentioned that you need to disable the OpenWrt DHCP server because you can only have a single DHCP server on a network.
If you are using a DHCP server that will provide service to multiple VLANs, you will want to search for "DHCP Relay" and you'll find other threads and packages that will help you through the process.