then you can give him a static address in the range/vlan he needs.
so if you MX is 192.168.8.1 and your first pc is 192.168.8.2 and your printer is 192.168.8.3 these should all work fine.
Then if you need to add another device on the network, you give them an address in the same subnet e.g 192.168.8.4
otherwise keep the address assignment to dhcp, and they will get assigned an address automatically
and what is he asking the static address for? for new device on the same network?