As a technical matter, 9066 gave the Secretary of War or military commanders under him the right to order the relocation. General DeWitt issued the orders for the evacuations, under authority of the Secretary of War. But that does not mean that he was the one who formulated the policy of relocating all the Japanese-Americans from that area. That was a policy decision and was the aim of 9066, as the pre-9066 policy discussions among FDR, Stimson, other cabinet officials and military commanders show.
FDR knew he was authorizing the rounding up of Japanese-Americans. DeWitt was just carrying FDR's policy. The idea of applying the order to the Japanese-Americans, en masse, did not originate with DeWitt. A military commander does not simply round up 120,000 Americans, including scores of thousands of native-born citizens, in the US proper, without the president's say so.