Had a similar problem. Option 66 is being passed but the next-server in the bootp wasn’t set. Try using the CLI on the fortigate to go in and edit the DHCP settings.

From the CLI:

FORT-310B # config vdom
FORT-310B (vdom) # edit <vdom name>
FORT-310B (<vdom name>) # config system dhcp server
FORT-310B (server) # edit 1 //Replace 1 with the number of the DHCP server id on the fortigate if more than one configured
FORT-310B (1) # [COLOR=#ff0000][B]set next-server 192.168.X.X[/B][/COLOR]
FORT-310B (1) # end

You need to set the next-server for the DHCP through the CLI in fortigate.

From the CLI:

FORT-310B # config vdom
FORT-310B (vdom) # edit <vdom name>
FORT-310B (<vdom name>) # config system dhcp server
FORT-310B (server) # edit 1 //Replace 1 with the number of the DHCP server id on the fortigate if more than one configured
FORT-310B (1) # [COLOR=#ff0000][B]set next-server 192.168.X.X[/B][/COLOR]
FORT-310B (1) # end