Chapter 11 IPsec VPN for FortiOS 5.0 : IPv6 Features : FortiGate interfaces can get IPv6 addresses from an IPv6 DHCP server
  
FortiGate interfaces can get IPv6 addresses from an IPv6 DHCP server
From the CLI, you can configure any FortiGate interface to get an IPv6 address from an IPv6 DHCP server. For example, to configure the wan2 interface to get an IPv6 address from an IPv6 DHCP server enter the following command:
config system interface
edit wan2
config ipv6
set ip6-mode dhcp
end
end